Light coverage isn't measured in "how many plants", it's measured in "how many square feet", because a "plant" can be almost any size at any given time! It all depends on how you are going to grow your plants (SOG, SCROG, small, medium, larg, or big friggin' trees).
A 600 watt HID is good for about 12 sq.ft. (more or less). You can start with one 600 watter (or something smaller), with the small plants all huddled together. And, as they grow and take more space, you can add another.
SOG or small, may only need one 600 watter. SCROG or medium could use two 600 watters. Large or "Big Friggin' Trees" could use three, or more!