I wouldn't plan on more than around 5-6 gpm on a 1/2" supply. Some of this is a restriction on the water flow velocity. If your pressure is quite high, you can flow a bit more, but you'll have long term problems as the faster the water flows, it can start to erode the pipe and fittings from cavitation and friction. Most 3/4" valves are capable of about double that flow. Depending on your pressure, you may not be happy with 2- 2.5gpm showerheads...depends on the design. More, is just asking for problems. Few of the valves have full-port openings, so act as a restrictor.