10 is OK
11 triggers the 'proximity memory full' message. On a Vista HCx, software 3.30/2.90
When more than 10 are loaded, the first 10** retain their proximity properties, and the rest still exist as Waypoints but without the proximity.
The size of the proximity circles, and whether they overlap of not, doesn't seem to affect it, I tried it with 200m (not overlapping) and 2km (overlapping) and it made no difference.
** in my trial, 1st 10 alphabetically. Didn't try a GPX where they might be listed non-alphabetically, and I would guess in that case, the 1st 10 in the GPX list.