If you're switching to the Onus Wunsler program, there's a sample of it on pages 300-01 of Starting Strength 2nd ed. There he uses 3 sets of 10, and that's what I've been doing. [EDIT: just looked again, and on some days the sample program says 5 sets of 10 - maybe someone else knows why the difference]
It's assistance work, so I'm not sure you necessarily have to make a strict progression with them - I generally use as much weight as I can tolerate across 3 sets and still feel a good burn in the back, which can vary depending on how the work sets felt that day. But maybe others have a different experience and have seen benefits from progressing them linearly like the other lifts.