How about defining ranges? The number is in flux for many people and they may not want to fix a number, but could be happy picking a range.

Something like:

Just starting - 5 watches or less
Small collection - between 6 and 10
Medium - between 11 and 25
Large - over 25, less than 50
Very large - over 50

Not sure what people consider a large collection really, just gave numbers off the top of my head! That consensus itself could be interesting. And not to look down on smaller collections at all, a tightly focussed group of the finest pieces is very worthwhile.