The easy answer is that you can use any pitch. Typically it gets more technically difficult as the pitch gets lower. 3/12 pitch is the lowest pitch that can have standard asphalt shingles and even that low can be problematic in some applications. There are low pitch options for roofs with pitches lower than 3/12. Installation, drainage, and maintenance are more of a concern with low pitch and/or flat roof installations. They typically are more expensive too. We have installed high pitch and flat roofs and everything in between with success. There are also aesthetics to consider and second story windows, which are completely dependent on your particular situation. Typically a 2 story colonial will be one of the most forgiving styles to accommodate reasonably easy aesthetic solutions.

I hope that helps!