You just make some shit up.
I like to use a 20-sided die that I referenced to a chart. After I finish my ME or DE lifts, I start rolling the die. Seriously though, there's no real rules here. When I visited Westside, Louie had me do about 10 sets of 15 reps of modified DB extensions and then when I was done with those he told me to do DB clean and press for like sets of 15...he didn't say how many sets, so I just kept doing them until he wanted to show me his reverse hyper.
However many you think you need. For me and the guys I train with, a typical ME/DE bench day might look like this for accessory work-
skull crushers off of the floor
side raises / front raises
other days we might throw some dips, pushdowns or pulldowns on top of that. We might drop the overhead press and do floor press or DB floor press or board press for some reps.
Really don't overthink it. You're probably at a point where doing just about anything will give you some nice gains. I usually tell people the following:
hit the triceps, shoulders and lats hard...if you want to throw in biceps go ahead. While you're doing this, think about how exercises are going to translate to your bench...if you bench 300 lbs. do you think doing pushdowns with 150 lbs. is very helpful? no, boards or lockouts are probably going to be a better choice, but everything has its' place.