Can I mention my undergrad ranking in CV, when there is no official document about it?
If you can't prove it, I wouldn't put it in your C.V.

For one, anyone reviewing your application will have no way of knowing if you're lying or telling the truth. That's never a feeling you want a reviewer to have.

Even more importantly: what if you were actually wrong? What if you were 8 out of 60? Or 6 out of 60? I trust that when you say you're 7 out of 60, you have a pretty good idea of that, but you can't be sure if your school doesn't issue an official rank.

For what it's worth, if your GPA is high enough, I doubt rank will matter much. #7 is great, but #7 out of 60 students is different than #7 out of 600 students or 6000 students. For that reason, the rank itself is less valuable than your actual GPA.