Ever DM a game where the PCs are up against a terrible foe and they want as much help as they can get, but you don’t want the fight to be trivial so you have every NPC they ask give them an excuse or say no? I challenge you to say yes instead, but only if it makes sense. If the players meet a knight who is the archenemy of the villain and he has an excuse that he can’t join them it doesn’t make sense. Maybe he’s injured—no matter, the cleric heals him. Now you have to say no even though the players know the characters backstory and know he would love nothing more than to gain vengeance. What do you do? The answer lies in encounter building, and I’ll go over the various methods you can use to allow the players to get NPC help without leaving your players saying “that’s it?” at the end.
Splitting the Entourage. Typically splitting the party is a bad thing, but especially if your party convinced a task force of NPCs to come along with them there might be a need for the party to split up to cover more ground. In this case I would strongly advise you send the NPCs off as one group and the PCs as another but if your players insist on splitting up it’s not the end of the world if they have NPCs along. What I try to do when PCs are in different places doing different things is stay in initiative order and go turn by turn so that I’m not letting one person do fifty things while everyone else waits. Another example of splitting the NPCs from the party might be if the vengeful knight is taunted by his quarry he may run off while the party is resting. Whether the NPCs are successful or not while separated from the party is up to you. If they are, they may claim some of the desirable treasure for their own. If they’re not they might become a liability for the party, perhaps even being slaughtered to the last to give the party a foreshadowing if the foe they face is of great power.
The Gandalf Gambit. Occasionally the party might want to have a particularly powerful NPC to come along. Sometimes it makes sense to say yes, even though you want to say no. Let’s say the party heads into a dungeon where they’re fighting orcs and they have a powerful archmage along that can take out a dozen foes with one spell. A wise spellcaster will reserve show of force until it is needed. Typically in this case I have the spellcaster focus on casting cantrips unless the party faces something they can’t overcome. If the party asks why isn’t he casting fireball every turn, I simply remind them that he didn’t make it to where he is wasting spell slots on small things. Anyone in the party who is or has played a caster will sympathize with that. In this scenario, maybe the orcs are being led by a balor and the party is only level 5 and have ignored all warnings that the dungeon is a death trap because why would the DM let you encounter something that might kill the party? The archmage might hold off the balor while the party escapes. A lone archmage isn’t going to be enough to hold a balor at bay for long, so they should tell the party explicitly not to stay and fight. On a side note, Gandalf is probably more like a solar than an archmage, but if you convince a solar to join your party you can be assured that they already planned to before you asked them.
The Side Combat. Sometimes you don’t want to employ the Gandalf gambit, because let’s say the powerful NPC is a melee combatant and let’s face it something like a balor will probably make short work of even a high level NPC without any sort of magical protection. In this case you can add a big (and I mean physically big with lots of hp, not necessarily high CR) monster or a bunch of small monsters to the combat for the NPC to handle. In this case you don’t have to track hp or anything for the combat, just describe how the battle is going every round. You can use this to raise or lower the pressure on the PCs depending on how their end of the battle is going. This tactic can also work if the party brought along an entourage and you don’t want them to split up.
Adopting Monsters. This is something I wouldn’t ever think of if my players weren’t in the habit of doing it. What do I mean by adopting monsters? Okay your PCs are in a dungeon and have found some evil creature that maybe was once human but was cursed, or maybe they find a clone bred to be a super soldier (both of these happened in the last couple of sessions in my own campaign). They inform you they want to take the creature along to try and cure it, and they want to give the clone a home where he won’t have to fight and will have free will. If you present my party with any villain or minion with a slightly sympathetic back story, I guarantee they will try to help them. Most often this will end poorly, but say your players encounter an evil werewolf and manage to cure it—one of two things might happen. Either they discover the werewolf has always been evil and curing it doesn’t change that, or it becomes a non-evil alignment that it was before. You don’t want these situations to turn badly for the players every time, but you want the player to succeed in what they want just enough so that it doesn’t cheapen the seriousness of the game by allowing them to make every bad guy good. On the other hand if you have a creature that is evil but only because it has never been treated good it may be possible to win it over but it’s not going to be an overnight thing. Such a creature will suspect treachery by the party and might seek to preempt its own demise by taking the party out. On the other hand, if it isn’t obsessed with it’s own survival, it might not see any harm in at least seeing what the party is going to do. If a creature is going to betray the PCs it might do so the moment the party is in combat—taking off and leaving the party behind or even attacking them. If they’re not hostile, they may not trust the party enough to fight for them, maybe even spending the battle sizing up the party. If they help out, it’s sure to be on their terms.
When Players (Attempt To) Recruit Armies. Typically a powerful figure with control of an army isn’t going to lend it out to help the players clear a dungeon, and large groups of any kind aren’t well suited to narrow spaces like tunnels. However, especially at higher levels, there may be more at stake than just a lost relic or a kidnapped NPC. The villain may be amassing an army to unleash on a city (if you’re also a Lord of the Rings fan you might know where I’m going with this) and perhaps the players need an army so they can focus on defeating the villain. Allow the players to reasonably convince a local king or lord to send out their army, but make sure the PCs know that this army isn’t going to do their job for them, it’s just going to buy some time or clear the way. When I ran Lost Mine of Phandelver, my party went to Neverwinter to petition Lord Protector Neverember for use of a military unit to help them slay a dragon. Incidentally one of the players (who had just joined the campaign already in progress) was playing a character that was one of the Mintarn mercenaries serving Neverwinter as a stand-in army. They succeeded in convincing Neverember to spare personnel resources, so being a clever politician he lent them the services of a single soldier, namely the PC that was under his command and granted him leave to travel with the party and help them with the dragon. I didn’t leave them totally without extra help, however, so they did manage to find an octagenarian dragonslayer in the city who was the sole surviving member of his adventuring party. He gave them pointers on how to best fight a dragon as long as they promised to take him along. During the battle they did well but the NPC, through no fault of my own, missed every shot to bad dice rolls. They took the dragon down pretty much on their own using a couple of simple pieces of advice and it was probably more satisfying and fun than bringing along 20 soldiers to absorb the XP (plus the old man was one of the most memorable characters for the party).
The more you DM the more you can think ahead. I try to encourage my players to do research if they’re going up against a powerful foe and they have the time, rather than round up a bunch of NPCs to boost their numbers, but if they really want help, and it makes sense, I allow it. The trick is to not allow it to ruin the challenge for the players and leave them feeling unfulfilled.