Having them all come on or off at the same time is fairly easy...it gets more complicated when you want to control them individually. If you have tiled walls, and are not doing a reconstruction, cutting holes in the tile to add new features is tough, especially if you can't access the back of the wall to run the needed new plumbing. So, it's kind of hard to give you good info, since your request is somewhat vague.
Another thing to consider is the amount of hot water you have - run lots of heads at the same time, and you can run the HW tank out fairly quickly. Plus, there's a definite limit on how much flow you can get through a typical valve, and you might need to upgrade the lines and valve to 3/4" or even 1" in an extreme situation. That's not counting the size of the drain line, which might no longer be large enough. There's a limit on the gpm a typical 2" drain line in a shower can handle, and an upgrade is required.