Since PB tracks these things by Innings Pitched and Plate Appearances, its nt the number of players as much as the number of IPs and PAs. I think the manual talks about this on the home page, but in general you will want to have about 650-700 PAs per position, and about 1300-1400 IPs overall - preferably 900+ as starters and 400+ as relief. Since the game allows you to use a player more than real life, a position player with 600+ PAs will probably be able to play almost every inning for you, and a starter with 180+ IP could go 6+ innings every 5 days.

For pitching, one thing on relievers is the recovery rate - many have only 2 or 3 batter's a day recovery rates even though they can face 12 to 14 batters at one time, which means even if they have a lot of innings pitched it may be hard to use them all as the pitcher will not recover fast enough.

I also like to have at least one or two INN pitchers available - this covers you for emergencies, and some INN pitchers make godo inning eaters in long relief and in blow outs...

Fred C