I am doing three right now but normally I do only two at a time. I just try to schedule them on different days and if they overlap, most of the time, I will reschedule one of them to push things out.  I found most of the programs'  routines are under thirty minutes so even doing two a day is only sixty min or less a day.  That is not too much for me but every person is different.

I did that for awhile but now I am doing just 2 at a time

I'm doing 3 of them at the same time, some days i reschedule a workout if i feel i need to. If you feel tired than maybe you should do that as well! It's all about how you feel and how your body is reacting!

It depend of the différents programmes ... you can do differents one if they dont use same part of your body (i'm a french guy my english is not the best you can see)

I've been doing two or three workouts a day as long as they don't concentrate on the same part of the body so I don't burn it out. I like to do the workouts one after another so I don't have to keep taking showers all day afterwards. When I need a rest day I just push the schedule forward so I don't skip anything.