I'm new at this, my first year (actually my first week!) and here's what I've heard from my customers: I carry about 65 different items and they compliment my selection, saying 'the other truck doesn't have this stuff'. I went through 3 cases (24ct) of Sponge Bob and 2 cases (24ct) of Snow Cones in 5 days. Nothing worse to me than telling customers "sorry, I'm out of that". I just ordered 8 cases of SBs and 5 of SCs. I don't plan on running out - would rather have left overs at the end of the season (but I doubt I will at this pace).
Some smaller van-style trucks carry 20-30 varieties for space, others believe if you limit the selection, you get faster transactions but I'd rather have a lot to choose from and have my teen kids (and their friends) help in the truck to create transaction speed.