Your back is just as important as your front, everything in life is about balance, you need to have a balanced physique so your body looks well portioned, if your back lacks density it takes away from your front, working your back properly will complement your full upper body make your chest look bigger and your waist smaller. On your back, Your lats, your middle back, your traps, your rear delts, and your lower back. If you use your head these muscle are all being worked during your other workouts as a secondary muscle so you can theoretically do away with back day and mix back day into all your workouts.

Your lats will be with chest day as they are being used as a secondary muscle when you are using incline bench. Your lats are used in pull ups and pull downs. Pull up/ Pull downs only get width in the lats, to get density we change are grip from palms away to palm towards you. Also for the lats we will do rows rows are also for density. On chest day we could also include your rear delts ( rear shoulder) as it is used as a secondary muscle.

Your middle back could also be with triceps day when you are doing close grip bench press for your triceps.Your middle back is really support muscle it doesnt have 1 purpose it has many middle back is used quite alot using many other excercises for other body parts. We are going to use they are not quite shrugs but they are defiantly not rows somewhere in between to hit this region.

Your traps are being used every time you lift a weight, best time to work traps is on shoulder day,
along with rear delts. For traps standing straight shrugs and incline shrugs so your leaning forward while doing your shrugs, this again is for density. For your rear delts we do reverse lateral raises

Your lower back will be used whenever you are working your core, your stomach. So dead lifts for lower back and sit ups for stomach
Definitely on all back muscle we are going for 20 reps with 1 min break before the next working set again it will be 20 reps so if you need to drop the weight do it, we be doing 5 sets 100 reps on each back part. 20 reps is definitely were its at for new growth on are backs.