12 BB ball bearing

This needs 12 steel BBs. Standard size for those is 4.57mm or 0.18 inch.

Either print the complete bearing with cage or if you want to print the cage in a different material, STLs of the races and cage separate are provided.

I didn't put a hole through the middle, use your favorite 3D modeling or CAD program to punch whatever size/shape you want.

To assemble, place the inner race inside the outer race.
Push the inner race to one side. That gives enough gap to insert the balls.
Evenly