I agree with everyone
these girls are a lot looser than you think, unless they crashed at your place for a month each and THEN slept with you.
however, this idea is pretty clever, it's genius, and props to the OP.
and unless she's a proud and open slut or considering a life in/of the church(and she has to be really young) then you can just assume her number is either
flat out lying
or being disengenuous(define sex as vag pen and nothing else, how many people have you slept with? now define it as either front and/or back? how about now? now definite it as 3rd base and past that, how about now? what is excluded, FBs? college/HS? one night stands? other countries? no one? does it count if you were drunk? high? regretted it? if it was with a friend? etc. these are all ways her number could be wrong without being an outright lie.)
EDIT: knew I forgot stuff
be wary of anyone who's from a third world country, spends a lot of time in one, or goes to them frequently,
being wary means use a rubber(not extra thin, those are for idiots)
also, hide/bolt down your valuables in some way, just in case.