I found 3 to be the best number, since i craft+gather alot, but two could do the trick too. Imo its more about the sale-space rather than extra inventory or ventures. Retainer 1 sales gatherermats, retainer 2 sells food/pots/everything else and retainer 3 sells 3* expensive stuff. Btw my retainers favourite name is peanutbuterjellytime.