There are 60 spots in each letter group, so you will board after, in order, medical preboards, A1-A60, families with children four and under, and B1-B42. There is a C group too. If you have a child four or younger, you can skip ahead of the rest of the B group (or at the very least that child and one parent can). If the flight didn't originate at your airport and started somewhere else, there may be people already on the plane before they start boarding at your airport. It's also quite common to have only a couple of people (or none at all) in the A1-A15 spots reserved for Business Select ticket holders and top-tier frequent fliers.
So, in all likelihood there still will be a few pairs of open seats together when you board. There may even be some pairs close to each other. You probably will have to go back farther on the plane find them, however.
I wouldn't let this worry you too much.