Progress first, volume later
As long as you are beating last week's weight or reps, your current sets are working. Adding sets prematurely just adds fatigue with no extra benefit.
Add a set when your current volume stops driving progress and you recover fully between sessions, not before.
Add a set when you have stalled at your current volume for a couple of weeks despite good effort, and you are recovering well between sessions. Volume is a tool you spend as you advance, so start on the low end and add sets only when progress stops. If you are still adding weight or reps each week, you do not need more sets yet.
As long as you are beating last week's weight or reps, your current sets are working. Adding sets prematurely just adds fatigue with no extra benefit.
If a muscle has plateaued for two or three weeks but you feel recovered and not beat up, bumping from say 12 to 15 weekly sets can restart growth.
Add one to two sets per muscle at a time, not a whole extra workout. Ramp toward the 10 to 20 set range over months rather than jumping there overnight.
Add a set when your current volume stops driving progress and you recover fully between sessions, not before.