Zams, this really depends on several things; how you wish to utilize this light, and what type of bulbs you use.
Generally, recessed lighting should be spaced no more than about 8' apart, and at least 24" away from the walls. Spot bulbs will give you a focused beam; nice for highlighting a particular area, but will not give very good "general" room light in a room with an 8' or lower ceiling. Flood bulbs will spread the beam more, and most will have about an 8' circle of direct light on the floor with an 8' ceiling. Ideally, you should place your lighting so that these circles of light slightly overlap.
In a 20 X 30 room with an 8' (or lower) ceiling, I'd use 12 fixtures in 3 rows of 4, beginning 24" from the end wall, with a fixture every 8'. One row down the center, and 2 outer rows being 24" off the side walls. This would give you an even spacing of 8' in any direction fixture to fixture.
You may wish to also consider furniture placement and room useage, too, as there really is no rule beyond even coverage. I used recessed fixtures in my great room, and placement was designed to balance on doors, windows, fireplace, but still basically complete even coverage and distribution of light. Joists will also govern how precise you can get, but the overall choice is yours.