It should be 30 per phase. If you wire it up as a 3-phase y, you can have 30A per each 120V phase, 3600 watts per phase, or 10,800 watts total (10.8KW). If you wire it up zig-zig or double delta for single phase, you should get 30A per phase, 3600 watts per phase, or 7200 watts total.

The current is normally limited by the wire size, and by how much heat (how high a temperature) the wire's insulation can withstand. So the current that can be drawn from an individual winding is the same whether the generator is wired for single phase or three phase. Total power is limited by how much current you can safely draw, and how much power the engine can provide. Unfortunately, I don't think you can create a generator (at least cost effectively) that can generate full power at both single and three phase.