The main game is 1 (well, if you want to get technical, it's zero), which gives you 255 mods, less however many DLC you have. Basically, every mod is given a number represented by a byte, which as an 8 digit binary number, can represent 256 discrete values.
It's because of how the mods are tracked or whatever (I'm sure there is a proper term that is escaping me). I'm at ~220 myself, I had no idea there was a limit. It also comes with AWKCR, and I'm not getting Weaponsmith, because there's WAAAAAAY to many weapons to download, and I'm almost at the 255 mod limit
