She needs to buy 3. If she only bought 2, she would have 14, since 7 multiplied by 2 is 14. But she would be missing one. And the invitations come in packages of 7, so you can't take a package and split it or something. So, even though 3 packages will be more than enough, she will need 3 packages. I hope u got this! If you need further help, then plz don't feel afraid. I don't bite! :]