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 distribute the balls and center the inner race.
Snap the cage into one side, between the balls.
Hopefully all 12 BB's can be inserted. Might need to change the design to a larger diameter so there's more space between the BB's.
Commercially made bearings are made using this procedure, with specialized equipment to shift the races, insert and distribute the balls then snap in a plastic cage or rivet or spot weld a two piece metal cage.