thats as vague as can be....

depends on:
what your selling/growing
what your time frame is
etc etc

With most plants 1 well pruned plant looks best... but who has 12months to grow/prune it to perfection? In many cases 3 plants work well to fill the pot fast.

dp

Sorry if I was vague, DeepRoots. These will be annual baskets. Petunias etc. I will be getting plugs and linners next week. I will plant the baskets for selling in mid to late May. Does that help? ;) I was thinking 3 would be good.

3's good....
hard to do a 1plant petunia basket, but it's possible.

if the plugs are really cheap stick 4-5, or in the past I've stuck 5 in a handful of pots and 3 in the rest.
That way some are ready earlier