There is a hardcoded number of ships that can be spawned.

However, if you wanted to get really mean, you could make one of those ships have an ability that spawns in additional pirate ships (that section of the gameplay.constants file uses the same system used by the Vasari Dark Fleet ability). Only thing is I doubt pirate ships spawned in this way will consider themselves part of the pirate fleet, so you might want to set it up so that it only triggers on planet bombing or something so that the extra pirate ships don't get spawned in half way to the target and just stay there.

As I consider this work around more and more, I think this might work.  Only, instead of on bombing the planet (as I'm usually successful on intercepting the siege ships), aiUseTime "OnlyWhenOrbitingEnemyPlanet" (like Embargo )

Are pirates enemies with the militia? I honestly can't recall but if they are then this definitely would not work.

Goa,

That was something I considered before settling on it, but for my purposes, I think it might be a consequence I can live with.  As I mentioned, I've gotten good at taking out Pillagers before they can even touch my world.  In fact, any player can do it.  All that needs to be done is to setup your orbital defense platforms so that the only part of your planet that is "exposed" is the backside relative to where the pirates would come in from.  This causes the pillagers to orbit the planet at the exit radius until they are at the point where they can then dive straight in at your planet and make it in this gap.  Makes for easy pickings.

For me, though, this mod is going to be used with some custom team maps.  So in the end, I'm willing to live with a portion of their fleet "deciding to stay behind" to attack someone else's world they just passed.

I'm going to do a file search, though, on aiUseTime, see if there's anything new in Rebellion that isn't listed here. (And as hard as I try, I can't get that link to actually stick you at the top of the page....)  I've noted a few things that are missing in that list for Rebellion.

WOEaintME,

Yes, Militia are considered as enemies to pirates.  However, unless the planet is actually colonized by the militia (vers being "NoOwner" with militia ships orbiting it), the ability won't trigger.

Fun fact I learned while experimenting with a buff that applies bonuses to the planet owner's allies' ships... if the planet has no owner, everyone is its ally....

Goa,

As a note of what I've discovered so far, the spawned vessels do seem to group into the pirate fleet so long as the spawner ship is still in the grav well and will jump with it.  It looks like, though, they act a little freelance while the spawner ship is still traversing the grav well.  It's like they're in the fleet, but without the fleet's specific orders and thus roam within range of the spawner ship.

But, I think I may have just found the perfect aiUseTime: "OnlyWhenAttackTargetIsInFiringSolutionAndHasAttackOrder"

It's used on the Charged Missile abilities for the Vasari LRM and Anti-fighter that disables their engines in return for greater range.  So long as they are attacking targets of opportunity while traveling to their final destination, it's not an attack order and thus the ability does not trigger.  Once there?  It's game on.