Never open signups and only let good people get invites, then look after those. This off course depends on if you really want a good tracker or you want one that depends on people you do not know donating.
OFF-topic but On-topic:
While you were thinking how many members does a tracker need, a member here was thinking how many tracker a member needs lol just ignore me
look at TL and demonoid .. .. more members , stuff gets seeded for longer time ..
FTN best in the business does not open for registration and it is only for VIP not for us.
I am proud to be a part of this communities.
It heavily depends on the targeted users, because the average size of a torrent is as important as the number of new torrents per day. For some standard 0day-scene tracker I'd guess 10k to have everything running smoothly. But it doesn't end here. There are trackers with a large user database but very little peers and then there are trackers with a large number of peers from even less members.
The quality of users cannot be easily measured and some trackers simply have better users than others.
As a few people have said, it all depends on content.
With 200 users who are going to all jump on a torrent, right there you are beating some of the massive sites on their worst torrents.
If they have already all grabbed the files elsewhere though, nobody is going to go to the smallest tracker for it.
Find something unique that appeals to the majority of users and anything over 20-30 users can work well.
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