If you are leaving skill cap settings at 600, I would say aim 5-10 people 'who get along' online at the same time.
That could honestly be anywhere from 30 to 200 accounts in my experience. A lot of transients to this game, just like 7DtD...
I had 12-24 online all the time for a few days, we had three thieves, it took time to find them, now I have 3-12 players online sporadically. It would only take one active griefer to ruin it all. What makes this game fun is also what makes it frustrating, it takes Investment and that can be ruined in a heartbeat.
I would strongly suggest building community slowly and making sure people know each other Before they enter the game.... I know it's not as likely to happen, but it is still my suggestion.