You can have as many live instruments as you want
your only limitation is the power of your computer, and of course, the amount of arms and fingers you have.
I find 8-12 tracks of audio and 2-3 live vst synths is about as much as I can physically handle without automation or getting in someone as a partner to do a live pa with me.
I think the secret to a live PA is making it sound like a DJ mix. A lot of PA`s go from kickdrum, to full mix, back down to kick drum, and then next tune repeat process. This kills the momentum in my opinion, so you don`t want to have too many synths and so on running at once as it will impede your ability to make good transitions from tune to tune.
The real secret of making a good pa, I think, is how you split your tunes down so they can go into ableton, and how you place them into the clip view.
I set my stuff out very logically so I can visually see the progression any tune would normally make.
I can then play any tune in any order, but always have a good logical visual structure on the screen so I can work out what to do next, or go wild and never get too lost.
Producion wise it`s no different to making a tune, all the parts should go in pre-eq`d so it all fits together in the mix nicely.
It`s too much for me to go into fully on a forum really, but I can take some screenshots I guess and wack em up so you can see my basic structure.
I am now doing 1-1 ableton Live PA sessions for people now though, so if you are really serious you can book me for a full day and I`ll show you the lot.