Brick 1 x 2 Ring with 1 x 1 Round Bricks

When building a round wall (as the yellow one in the pic below) is there a fixed amount needed for me to be able to build on top of it. At first I used 15 1×3 bricks and 15 1×1 round bricks. This made it very difficult to built on top of. Now I’ve expanded it to 16 bricks of each and it’s nearly 21 studs wide.