At least 8 teams
https://www.espn.com/nfl/story/_/id/31559189/usfl-relaunching-spring-2022-least-eight-teams
Can't help but think this won't work. Spring football hasn't done particularly great and is bordering on oversaturated for the amount of talent available. Having all the old teams and logos is cool enough but will that really play for anyone under 45? I'm 35 and I remember World League/NFL Europe but certainly not the USFL (NFL Europe teams and logos/helmets were way cooler IMO)
Also, the USFL predated free agency if I recall, so that's how they were able to get some elite talent. They won't be able to do that now.
I'm excited to watch at least as much of this as I did of the original USFL.