As a general rule specific Pirates crews show up in between ranges of floors like say the poopy pirates™ will show up in between floor 40 and 69 (I made that one up).

But it's also random if they show up or not, I recommend looking up a list online if you're hunting for all of them.

Each type of pirates have two conditions: what floor they first appear on, and what type of item they are more likely to spawn in. All pirates that hold maps are unlocked by floor 60, so if you have a legendary item of any rank you can use it to farm pirates once you get down that far. There are two other pirates that do need a top rank item to spawn, but they don't appear until you've got all the maps and defeated Baal so that's a long way off.
As for item type, I didn't find it to be as important, since there are so many pirates that it's still hard to get the one you want. Though if you're struggling to find a certain pirate, there is a list for that, for instance, Red Pirates being more common in shoes.
Remember also, equipping maps you find to your characters also raises the pirate spawn rate in general. With all maps, nearly every floor will have a pirate if you wait 3 turns.
Have fun pirate hunting!