probably like 10 would be my estimate doing some quick math.
its hard to tell man.. so many different things depend on it.
i.e strain, soil, nutes, proper air flow, proper heat, enough space for the plants to have room to all get the light to reach them..
i think it would be hard getting two 600watts to cover 30 plants. probably would be better off doing 6 200watts to cover more areas.