Like CDR alluded, a better question is "what rpm do you start your sprint in?"
Even then, the biggest factor will be your personal preference.

Pick the appropriate gear to start your sprint at a given rpm. The gear is the variable; it will depend on the conditions leading into said sprint.

Added in edit: FWIW, I never look down or consciously think about what gear I am in when I sprint during a race. I just pick a gear such that I launch from like 90-100 rpm.