I use three, but it starts to get tough after two. I started with Brawler, then picked up Ravenous after toying around with Kitana. I just couldn't get over how cool Mileena turned out in this version of the game so I wanted her to be my #2. She took a lot of work before I got comfortable with her, but I got there.
After awhile I thought a 3rd for funsies was in order. People were starting to think Kenshi was looking better in early XL, so I thought I would look into him even though I'm stranded on PC. That was fun for awhile, but then I started using D'Vorah SQ just to try and understand her and ended up with an unexpected character crush. I spent a lot of time trying to get the bug cancels down, with some uneven success. I use her a good deal of the time now, bumping Mileena down to #3 and Kenshi out wandering about somewhere in the land of abandoned characters.
I find it really hard to feel like 3 is as practical to keep up on as 2. Once you learn a character you always kind of do know them, but it's not the same as using them regularly, and I'm pretty sure 3 is my max for keeping comfortable.
I don't ever use multiple variations. I'm glad they are there for me to choose from, but I prefer to have totally different characters in my roster as opposed to using versions of the same character. I need the variety, and I think I would get too easily confused as to what I can and can't do when shifting variations.
