I have to slightly disagree with Neily on having a Heavyweight Championship. I did that when I started Underground Wrestling and after about 6 months of shows I realized it was kind of ridiculous having a Heavyweight Championship when 90% of the LOTF guys aren't heavyweights. So instead of the Underground Heavyweight Championship I just had the Underground Championship.
Anyways. Decide on how often you want to run shows. Set up a tournament and then go from there. Your first angle could be the loser of the title match wanting a rematch. That's fairly simple.
When to add and subtract guys. I'd say run it in real time and just add guys as sets come in. When to subtract? I never subtract personally. I just move guys down the card and make them less important if I run out of things for them to do or get tired of them. I'd say just axe guys when you get worn out with them. And bring them back when they seem new and fresh again.
Rankings. That's up to how much book keeping you want to do. I usually go by who's hot at the moment. 2 or 3 wins in a row for an LOTF guy is impressive and worthy of a title shot. But there are several ways to do rankings.
You could do an NHL style point system. 2 pts. for a win. 1 pt. for a tie or a win by DQ or count out.
The Chikara system where you have to win three matches in a row to get a shot at the belts.
Straight up wins and losses and whoever is at the top of the rankings gets a shot.
There are lots of ways to do it.
The number of belts depends on you. I have all of the LOTF cards going currently and the NOW set. I currently have 4 belts and a trophy going around. And that's pretty comfortable for the number of guys running around. It's really a ratio game with belts. If you have enough challengers to keep it interesting then go for it. Just remember that if you only have one singles belt that the low end guys will have nothing to shoot for.
Start simple with belts though. Just have your main belt and the tag titles to start. Let those get established for a few months and then see what the landscape of your promotion looks like after that. You might decide you want more and you might want less.
As far as number of wrestlers. That depends on a lot of factors. I'd say you'd want between 32-40 wrestlers. That'll keep things fresh and give you a deep roster.
I would also recommend adding in some of the LOW guys that do a lot of indy work. the Sandman, Sabu, Chris Candido, Steve Corino, Danny Doring, Justin Credible, Brad Armstrong, D'Lo Brown, Al Snow and several others mix in well with the LOTF guys.