20 sets is a bit too long unless you are only doing chest , then you cannot do shoulders or triceps till 3 days later. anyways 20 sets is a bit too much i would do 16, You need 48 hours at least to fully recover from a good workout so if you are doing one muscle per day you can do: (except for triceps and shoulders since both need pressing)
1- chest 16 sets
2- Back 16 sets + rear deltoids and traps
4- shoulders (9 sets) without rear deltoids and traps + triceps 12 sets
5- biceps 12 sets
6- legs (16 sets)
or if you want you can do a muscle twice a week but i would do instead of a total of 16 sets split in half 8 and 8 ..i would do once 10 sets and the next time 6 sets
1- Chest 10 sets- Triceps 9 sets
2- Back 10 sets- rear deltoids 3sets-biceps 9 sets
3- legs 12-16 sets- Traps 4 sets-
5- Chest 6 sets- shoulders 9 sets -triceps 4 sets
6 _ back 6sets- biceps 6sets
this way you have : a total of 16 sets for chest and back , 13 sets for triceps, 15 sets for biceps and 16 sets for shoulders....you can do abs with legs or any rest day .
This is a pretty tough workout so it is better to take 2 days of rest in a row if needed.