It all depends on the spread of the light, you can get halogens with a 20 degree spread upto 45 degrees. Normally you calculate the square footage that the plamp will cover then plan how many you need for your floor plan. I would also look at wall lighting as well as ceiling, your idea of...
Judging by the 21' room length, i presume that your are going to have two rows of seats, if so, you may find the 119" screen to high for the second row of seats on a riser to see the bottom over the first rows heads, also, if you mount the PJ on the ceiling, as opposed to mounting in a bulkhead...
What I need to know is can I put the speakers in the ceiling? Only if you have to, and then only the rears, in ceiling speakers throw out the surround effect. What kind of wires should I run for future projector in the ceiling? HDMI, Component, Power Or better still, just a big conduit...