How many different excercises should be done in one session if I plan on doing two waves for one exercise. Example: for a bench press, I would do a set of 6, then 5, then 4. Then repeat. This would go for every exercise. Should a two day split be done with push/pull or upper/lower?
I never go beyond 6 per session
Really depends on what you are doing, trying to do etc. IMO. I do as many as 12 when running an HST routine, or as few as 3 when doing something like my current 5x5.
For me, changing things up definitely works best. If the number of exercises increases, the number of sets goes down, sets up, exercises down etc. etc.
I would say 4-6
That really depends on the volume, nature of the exercises and your recovery capacity. Big compounds like squat and deadlift are not in the same game as bench and curl when it comes to fatigue. You'll have to play with it and see what works best for your goals.
I wouldn't wave-load all of your exercises in a session, but that's just me.