Should be the BMP from what we’ve seen. We’re only shown 1 other island for Kaido besides Wano. That’s how much Oda didn’t give a damn about the waste Beast Pirates.
WB Pirates were gassed up but never actually shown…and logically if Blackbeard took over their territory as claimed they should be similar in size.
Shanks seemingly has mostly fodder that could be wiped out from 1 attack by Eustass Kidd.
Cross Guild is just a joke to even include, and the Strawhats Grand Fleet sports 5,600.
As I’ve always maintained, the BMP are the best when it actually comes to feeling like an empire.