I'm going to be doing 4 Sets, for 12,10,8,6 Reps
How many exercises should I do for say like
Chest
Legs
Biceps
Back
Shoulders
Depends whether you're doing body-part, upper/lower, or full-body splits.
Generally, you would select an exercise that hits each area/body-part.
So for each body-part you would want to make sure you did an exercise/s that targets the following:
Chest - upper, middle (lower - i.e. decline - is not necessary)
Legs - quadriceps, hamstrings, glutes, calves
Biceps - long head, short head
Back - upper lats, lower lats, rhomboids, erector spinae
Shoulders - anterior delt, lateral delts, rear delts, traps
Generally 4 exercises per session if doing a body-part split is enough. However, if you're using compound exercises, then some of them will target a lot of muscles, and you don't need to do as many.
Re biceps, this is not necessary, since most back exercises hit these; and most chest exercises also hit the triceps (unless you're doing on a separate day).
And not all body-parts would require a 4 x 12, 10, 8, 6 set/rep scheme, and would suit others, depending on your goal.
Look around; there are many different training programmes utilized by members here (as well as in general). You need to design one to suit your training goal. If in doubt, you can always get a personal trainer who should write an individualized programme to suit your specific goals and needs.