I think 100 is perhaps a bit of an exaggeration and it matters in this case. Subbing to perhaps 5 communities about Formel 1 cars or whatever have you is not so much of an ordeal. Especially because probably most users in these communities will also be subbed to the other communities. So you very likely won't need to post the same stuff multiple times.

It would be great though, if communities would group better in some way, perhaps like tabs in my own dashboard where I can sort the communities by topic.

It would also help if people would use the community browser to see if there are similar communities already to the one they plan to make, so that they can connect with each other.