Assistance is assistance; it’s used to balance your training. I lift twice a week and typically pick my assistance for the day, that day.
Jim now recommends doing assistance from 3 categories - push, pull, and single leg/core. I just cracked open my copy of 2nd Edition and see the 4 movements for day one and also on day two. He says pick two of them. I would change it up a bit and pick 3 movements, for a total of 25-50 reps during leader, and 50-100 reps during anchor. You should pick 1 push, 1 pull, and 1 single leg or core exercise and aim for a total amount of reps.
Edit: you can also choose to do no reps from a category if your volume in supplemental is high - if doing BBB for squat might not lend itself to doing single leg squats as assistance the same day. Same for bench, you may do BBS in bench and not want to do any more pressing that day and skip out on push assistance. Just make it for that day and don’t get to caught up in the little things. The main lifts are what’s important, the assistance is there to keep some balance in your training.