Okay so I went back and opened it up again, and yes, there was still a lot of space for more balls. So there was the original 20, and the extra 2 I'd added earlier, and enough space by eyeball for 4 more. That would bring it to 26 from the original 20 + cage. Seems to have come together okay, at least from the looks and feel of it.

I guess the worst that could happen is I put in too many and end up having to replace the headset ...

This was very educational. Thanks.