Short answer: Go for a server with fewer than 10,000 members.
Long answer: It's not so much about numbers as percentage of the network. If everyone is on a small group of giant servers that will cause severe problems now and in the long term.
So, if you look at the numbers currently on https://joinmastodon.org/servers (in the top right corner of each server's info box), try to go for servers at the smaller end of the scale.