As others have said, in all likelihood there will be enough seats if the airline is willing to sell tickets to you. Despite what the seat map shows, it's quite possible that they really haven't sold all the seats yet. And even if they have, people change/cancel tickets all the time and no-shows and misconnects really do occur, so the load might be just about right by the day of the flight.

If the flight really is overbooked and you can't get seat assignments in advance, you might want to check in online. Most airlines will permit you to check in 24 hours before the first flight, and checking in earlier might reduce your odds of being involuntarily denied boarding. The other advantage of early check-in is that you may get better seating options -- like exit row seating, or at least scoring two seats together.