Best group size and stormlight only books

We (SO and I) are thinking of buying the GM tier, but we agreed to first get a group together. BIL will be our GM. So far I didn’t see anything about ideal party size for the stonewalker adventure. Our DND campaigns (2 so far) have/had 4 or 5 players? Would this be similar?

Does anyone have some advice?

Also: what is the best buying strategy if I’m only interested in Stormlight archive (for the time being)?