How many screws do I need in my subfloor?
According to Table R602.3(1) of International Residential Code (IRC), subfloor fastener schedule is as follows:


3/8"-1/2" panels. Use 6d common nails, spaced 6" apart on edges and 12" apart in the field.
19/32"-1" panels. Use 8d common nails, spaced 6" apart on edges and 12" apart in the field.
1 1/8"-1 1/4" panels. Use 10d common or 8d deformed nails, spaced 6" apart on edges and 12" apart in the field.


NOTES:  


You can use screws (and I recommend you do), however, they must be equivalent to the nails listed in the code.
"edge" refers to the perimeter of each panel, while "field" refers to fastener not at the edge of each panel. 
If you're building from a kit, you should follow the manufacturer's installation instructions.