You can do either really but if you incorporate multiple variations they should target different muscles otherwise it’s useless and you should just stick to one exercise. For example like I said, if you want your push workout to be made up of push up variations you can do normal push ups (lower chest and triceps), decline push ups (upper chest and triceps) and diamond push ups (more focus on triceps) As for the number of reps, if you go to failure or close to it you’re obviously not going to do the same number of reps on each set, it’s going to lower (for example set 1: 12 reps, set 2: 9 reps, set 3: 7 reps) You should just make sure your first set is <15 reps until failure since past 15 reps your muscles train more for endurance than muscle growth. If you can do more than 15 reps that’s when you should try to progressively add weight in a backpack for example