If their presence makes sense given the surrounding fiction, I've got no problem with this.
But if the PCs have reason to believe there's only five plus the captain on board, having an extra five come up from below-decks doesn't make sense; even less so if their presence would have been somewhat obvious due to the boat riding lower in the water than it otherwise would have.
And that level of fanatic support is something the PCs could learn about beforehand, and prepare for. Again, fine with this.
I don't know what that means.
Is this functionally the same as D&D's Monster Summoning spells? Failing that, where do the extra raiders come from and-or where were they before; and could the PCs have known about them ahead of time and thus maybe decided to take them out first?
That's up to the GM and players to decide. Maybe they were hiding below decks the whole time. Maybe a second boat appears out of the fog--far range is anywhere from 30-100 feet away, but you can always say they come from Very Far or Off The Battlefield, as DH puts it. That would just give likely the PCs some extra time to prepare (assuming that one of them didn't roll with Fear and give the GM a chance to take the spotlight, as per the rules). Daggerheart isn't the type of game where everything needs to be spelled out
exactly; it
wants the GM and PCs to work together to determine the story.
Could the PC have known about them ahead of time? No. It's not that type of game. But if the GM doesn't feel like it makes sense for a pirate horde to appear--like it's been established that the Pirate Captain is really alone, or the PCs have taken actions which would prevent the Captain from calling for help--then they GM shouldn't use that ability.
If the extra baddies are already present somehow in the fiction and can potentially be learned about by the PCs, all is good. Having them show up out of the blue "just because" is bad. If for example the PCs do some pre-scouting and determine there's ten baddies camped out in the dockside warehouse and five more plus the captain on the boat, then they've every reason to expect that's what they have to deal with.
Or like here, where there can be
plenty of other places where extra baddies can be stored. It's unlikely that this is the
only warehouse around, after all. Imagine a movie where the Captain seems to be alone, only for a dozen corsairs, curvy swords clenched between their teeth, pop up from behind all the rocks, foliage, and debris.
Which means if they sneak into the warehouse and quietly knock off the ten in there before pressing on to the boat, the warehouse can't provide any reinforcements...unless there's a secret area below it, but then the PCs would in theory have had a chance to find that too.
The problem with adding extra baddies on the fly "just because" is that all too often there should have been some sign of their existence and-or presence earlier, except because they didn't exist yet in the DM's mind no such sign was provided. And that's not fair to the players.
First, this assumes that the players are on the lookout for these signs to begin with. From looking at a pirate vessel, how would you know, just from the signs, if there was five pirates or ten? Ten beds? On a pirate ship? Nah--they'd be hot racking it most likely. (My super-quick research suggests that in real life, it would be anywhere from 100-200 crew on a pirate ship). Carefully counting them? Sure, the PCs can do that, if they want to take the time (and during combat where everyone is running around is
not the time--although I'm pretty sure there was a spell somewhere in 2e that did exactly that).
The same is true for any sort of encounter. How do the players actually find out how many goblins are in the lair? Watch the entrance for days and hope they don't get caught? Knock on the door and pretend they're census takers?
Fortunately, in Daggerheart, the GM often has to spend a resource for a leader to summon aid like this, or has a limitation on how often they can do it (or both, like with the Pirate Captain here). Even if the Captain wasn't limited to one summoning per scene, they still only have five stress total, and there's plenty of PC abilities that will reduce that number. And despite the name, a horde in Daggerheart is treated as a single entity, and not a terribly strong one either. There are some leaders who can summon freely, but all of them (I think) can only summon minions, which not only has only one hp, but a single blow may even take out extras who are nearby.
So it may not seem fair, but the fairness is actually built into the system.
Now, (A)D&D doesn't have those sort of restrictions. However, I don't think it's necessarily unfair unless the GM is being actively antagonistic, and they can do that even
without sending in more minions, without cheating, just by playing the adversaries intelligently and giving them what they logically should have access to--which most GMs
don't, because they tend to think of the adversaries as monsters who exist to be killed and not as people who have spent generations having to deal with home invasions.
And while some people will say "Oh, so it's OK if the players don't know what you're doing?" how
would the players actually,
really know how many pirates there are to be able to claim it's "unfair"? Did they read your adventure notes?