Just because you selected a number while checking in online doesn't mean thats all you're going to get.
If they really are oversold then they'll be offering much higher amounts at the gate... particularly if it means another overnight stay. You could then also ask for taxi/cab voucher and hotel along with the bump voucher.
Regarding the ticket, I am assuming as others said, that you have a basic economy ticket and that is why you don't have a seat assignment?