Can you have only one single gental wart? how contagious is one?

Number is not the : definitive answer. Once exposed you should be treated and carefully followed by your physician.

Possible, unlikely: Genital warts usually are multiple, but single ones are possible. But don't assume it's a wart if uncertain: there are several skin conditions that mimic warts. See a doctor for diagnosis, and for sure before you try to treat it yourself -- which would interfere with accurate diagnosis later. Whether multiple or single, genital warts and other HPV infections are contagious by sex.