Calculating power requirements of components
You can see how much current you can draw by looking at the datasheet of the IRF520 device used. The IRF520 can have 9.2A @ 25 degrees, but you should never reach this number since its really an absolute max spec as well. 5-6A is likely a more safe bet and it depends on the layout of the device and its power dissipation.

So, with 10W bulbs, each drawing 0.416A, you could have around 14 lamps.