Currently the max coop upgrade is 8. I want to propose the next upgrade tier that costs maybe 200k for a coop limit of 10.
Now you might ask yourself why? For me, Meep Skyblock is about the adventure you have with friends. There are few more island members I'd love to play with, but the limit is preventing me from adding them.
Some might say that an increase to 10 members would give an unfair advantage, but I disagree. Having more island members means spending more money on the island coop upgrade path. It is money that we can no longer get back. Currently missions have a hard cap on the amount of money you can earn, so us spending it on the coop upgrade would cancel any money advantage from extra island members.