Should I ask this girl for her number or ask her out?

she sounds like she likes you quite a lot - dont let the fact she is hot and popular put you off because a lot of attractive girls dont actually get as much attention as you think (guys are often intimidated) the other guy is probably just a friend of hers. if you are acting friendly then its normal to have her number anyway so ask for that first. maybe leave it a few weeks and if she is still acting interested ask her out . in person though