Guppy breeding question - how to insure genetic diversity?

I've decided to try establishing a guppy colony in my 29 gallon tank. I'm hoping to keep the same line of guppies alive in my tank for the next several years at least. I don't know a lot about breeding so I need to know how many guppies I need to protect against inbreeding. There are three stores here in town and I'm pretty sure they all have different suppliers since they all seem to have completely different types of fish in stock all the time. I thought if I picked up a couple of males at one and a couple of females at another one or a male and female at each one or something similar I could work it out to have a pretty genetically diverse colony at first and avoid a lot of problems that inbreeding can bring. Approximately how many fish do I need to begin with?