We only have 3 Bettas now, but are going to get more.. it's true, you can't have just 1.

There are so many Bettas to be rescued at places like petsmart and the even worse petco. Even though we want to get Bettas from Miami Aquariums because of their reputation and love of Bettas, we still are going to rescue the worst-off Bettas from petco (that are sure to die in the store because they look so bad off & people will pass them by), do our best to nurse them back to health and then find them good homes (making sure they get nice tanks). I wish the manager of our petco wasn't so cheap that he won't discount sick or almost-dead bettas any, but it will be worth it to save the ones that we can.
You could get a good tank divider for the 20g and have room for 2 Bettas.. just be sure they can't get over the top of it if you do.