Smart guys say it's hard to program assistance for other people. Everyone can handle different amounts, based on what kind of condition they are in. You may need to experiment a little.
If you've run a steady program like 5/3/1 for a few cycles, you probably have a "feel" for it. You understand the training max, how hard to work, and how the work outs build on each other. The way you should feel to make progress without Overtraining/overreaching.
To figure out how many reps of assistance you can handle, use the total reps approach. Add in your assistance lifts, start light and do the 15-30 reps per exercise you planned. Then, instead of adding weight, at to the rep totals.
Week 1: 15-30 total reps for assistance exercises.
Week 2: 35-45 total reps
Week 3: 50-60 total reps
Maybe you'll even get to the 100 total reps Jim recommends. This was too much for me. I was making progress with 40-60 reps. I added reps, but started to feel run down and beat up once I got around 70. My "real" lifts would suffer, so I'd tone it down.
I stuck with 3 x15.
Then I started to add 5-10 pounds to the bar. Progress was good, until I got too heavy. Then my joints would start to hurt and my "real" lifts would suffer. So I slowed down the progression.
Now it's like
Week 1: Rows 3 x15 with some light weight
Week 2: Rows 3 x20 with that same light weight
Week 3: Rows, drop reps back down to 3 x15, add 5-10 pounds
Week 4: Rows, 3 x20 with that heavier weight.
As long as I go steady, the main work rolls forward. If I get out of line with the assistance, it shows right away on the real work. My body tells me Exactly what to do, if I listen.