As many as you can put in there (in theory). It really depends on how you space them, what species you get, and how willing you are to regularly trim fast growing corals.
Well, now I'm committed. How many corals you intend to put in a 16 gallon will depend on what type they are, how much water flow you can afford them, how much light you can get throughout the tank, and how much surface area you give them to grow on. Acropora you could fit maybe 12 or 14 sticks without overcrowding the tank, giving them room to grow into without shadowing themselves or others, or blocking flow. Torches and hammers I'd limit to two or three, as well as anything that has long sweepers. For a mixed reef style I would do 1 euphyllia, a couple of micromussa or favia, being sure they can't 'reach out and touch' their neighbors, three or four acropora, millepora, or birds nest corals, an encrusting montipora and a branching monti, probably a forest fire digi.