Assistance is just that: assistance. It isn't the biggest deal. Generally I'll do three to five sets of eight to 15. Right now, it's 5x10 - Jim Wendler says if in doubt do five sets of 10 and that's good enough for me. I never take them to failure. I don't see the point. They're just there for balance and assistance.
BTW, I always write setsxreps, so 5x10 is 5 sets of 10 reps.
I think generally one to three assistance exercises is plenty. For you, because you want to get bigger thee is probably good. I'd go one pressing exercise and two rowing exercises. That way, you can have a bigger, heavier row followed by a lighter one.
Bench press - main exercise however you want to train it but I'd recommend 5x5, 5x3 or something because irs easier to practice technique with lower reps, plus you'll be able to use slightly heavier weights (75-85%)
Press assistance - 5x10
Heavy row - 5x10
Light row - 5x15 - you could probably take the last set to failure if you want.
Then on the other days, where you just do 3x5 at 65-75% bench to practice just add a few lighter sets of rows after. I am convinced of all assistance, rowing is the most productive across all the big lifts. It's helped me hugely.
The eight sets of two or three were when I did dedicated speed work. It worked, but isn't necessary. It was more a way to do bench but without it being too heavy. It definitely isn't particularly helpful for size, but if you do want to add volume to a lift it's an option.