Private ones too. Privacy in private trackers is largely a myth, and you should be using a VPN regardless of public or private.

If you (a relative Joe Schmo Nobody in the torrenting scene) can get an invite to the private tracker, you really think a billion dollar media industry couldn’t arrange to get one too? Of course they have straw-man accounts on the big private trackers, and of course they’re quietly seeding media to be able to log IP addresses that connect to the swarm.

The only real benefit private trackers have is better seeding requirements (meaning stuff typically downloads faster, and is less likely to stall indefinitely) and better request systems (meaning obscure media is usually easier to find, and you can request media that is missing).