Custom map? The pirates tend to have a fair force at their base, but 70-100 of each type is absurd. I also don't know of a map that has a pirate base as the only way across.

Any completed pirate raids after which they have ships remaining return to defend their base. So if you had a bunch of raids that had ships left over, that's them It can get to some pretty interesting numbers of ships. Thankfully, they don't have shields or shield mitigation, so they are huge suckers for AoE damage.

just tell me wher to click to make screenshot
i will show a REAL pirat planet.

I guess Annatar11 is right, all the pirats went to attack the AI and guess AI never put much of resistance so they woudl prabobly attack some small base blow it up and return

Anecdotal, but I had a medium random single map last night vs 2 ai that had the pirate base as the only path between me and the 2 ai opponents. In this one scenario only was the only time I saw MASSIVE fleets by the pirates, and I could only assume it was because the ai made a fleet to go after each of us, making 3 large fleets, plus their home grav well protection fleet. All 3 of us (me and both ai players) were listed in the pirate window as being targeted by them for the attack. Tghere had to be, easily, 300+ ships in the pirate home grav well at the time I peeked in... Yikes!

vs a single player and a single ai on a small map? Very curious...

Had it happen to me once, and the fleet did get huge.. Choke point for the whole map, had 4 phase lanes in, two for each side. Made it easy to expand on my side !

Actually, they do "grow". During a pirate raid, the pirates spawn at the closest pirate base to their victim. Therefore, if you never had them come after you, there would be hardly any ships at that pirate base, but lots at the other one.

No Or at least, it's not supposed to

I didn't know that. Thank you.

Ah I was too lazy to quote the old ones.

Hit and run tactic, pure and simple. Build two sizeable fleets consisting of only capital ships. If you have the resources, build two fleets each with three main battleships, 2 carriers and one support. Jump in with the first fleet starting at the left/top of the phase lane. Position the other fleet at the right/bottom of the phase lane. After 15-20 seconds, jump your first fleet back to your starting point (using Shift+R speeds things up!) and jump your second fleet in.
Make sure your point of departure sector has repair stations in the gravity well, to quickly ready your fleet. I found that building 2 or 3 repair stations speeds things up enough to get your first fleet in top shape fairly quick.

When your second fleet has taken a few hits after 15-20 seconds, retreat them back and send in the first again, etc. Manage your attack fleet's special abilities personally, I found that the fleet AI sometimes uses the abilities at rather silly moments (repair cloud when everyone is at 100%hp for example). When playing Vasari I recommend the support cruisers ability to add mobility to all of your cap ships when retreating.

This all depens on lots of research in warfare (more armor/shield regen/antimatter reserve-regen/better guns), and a minimal of level 4 cap ships.

I know it is tempting to throw everything in at once, but forget that, send in your main guns first, with your support a split second behind them, the Pirate AI should focus on your main ships asap. If you are playing Vasari (can't remember if any other race got this ability) send in a Colony Capital ship as well and use it's ability to drop the armor rating of the pirates.

As you can read, I favor the Vasari and their Capital ships greatly.

How can there be so many replies, and not a single right answer???

Just guessing here, but you have the 'He's no good to me dead' achievement, don't you.

The pirates will increase their forces with every cent of bounty they collect!

SO if you get into a bidding war with an opponent, suddenly the pirate rating jumps from low to extreme, and a horde of pirates swarms the loser. What you end up with is 100+ cutlasses and corsairs, which on a small map takes forever to destroy.

Whereas if you just IGNORE bounties as much as possible, the total number of pirates per base is about 40ish.

Couple of space buccaneer facts:
*Pirates will not jump star systems, this means that if you take out the pirate base in your home system, but there is a base elsewhere, the pirates will only attack planets in THAT(those) system/s.
*You don't need to take out the base defenders to stop the raids, just the planet. (5 novaliths is awesome for this). But culture does zip.
*Pirates have bugger all fighter/bomber defenses. (Read the Percheron desc. for background info on this) Carrier capital ships work very well with this, or a Percheron/Aerie/Lanstrata swarm with an Light frig screen.

As for dealing with it once you have mucked up the pirates' base/s, the hit and run will work a lot bettr once you stop replenishing the ships with bounty:
1. Stop using bounties.
2. Use a siege frigate swarm to take out the pirate planet, or novaliths if you're that lucky.
3. Hit and run with light carrier swarms. Use light frigates in front to draw fire, with your carriers tucked away outside of the well. When the light frigates are kaput, retreat the carriers, and repeat.

Actually, the only answer that's not right here is yours because it doesn't address the original question.

Yes, the amount of bounty helps determine the strength of the pirate's raid (but it's not the only factor). But the original question was why there are so many ships at the pirate base itself. Which has nothing directly to do with bounty, but only surviving ships returning to the base.

Indirectly, the higher the bounties the bigger the raid so the more ships may survive, but bounty is not a direct effect on the size of the garrison fleet.

Yup. Start a small map, don't outbid the AI, let the first pirate wave take out everything around your planet, and then look at what the ships do They stick around for a little bit, then go back to their base and sit there.