If you're talking about residential, there is no NEC limit. I've always followed the common sense rule of 8 on a 15A circuit and 10 on a 20A circuit in residential. It gives you room to add on to a circuit later. If it's a commercial setting you would figure 180VA per so it would be 10 on a 15A @ 120V and 12 on a 20A @ 120V