I just took one apart the other day, the type with a clamp on-top of the so-called top-nut.
The bearings fell out and I picked-up 41, so I am thinking that one was lost or missing.
The balls measured 0.185 or 0.186 so maybe 3/16ā nominal; (or maybe 4.7mm metric?).
I could fit 22 in the top race, but maybe only 21 felt just right?