There are two schools of thought. You spell out numbers one through ten, and type out numerically 11 and up. The other says you spell out numbers one through one hundred, then from 101 up you write numerically. The Chicago Manual of Style goes by the latter. Associated Press has different answers based on money and other issues. I tend to lean toward the Chicago Manual of Style, but it's your choice to do as you please. Often times, it's writers who are more concerned with details like those, more so than readers.