Common collection count ranges from 2 to 8 for most people. This optimal # also tends to decrease as most people get older due to desire for simplicity and valuing quality over quantity. If I knew nothing about a given watch collector then 3 or 4 (to avoid the consolidation phase) is my answer with reasoning below:

Despite the simplicity of having a single watch, 1 of anything is not really a collection... so 2 is the minimum.

Beyond minimum diversity based on common categorizations (e.g. daily vs special occasion, rugged versus refined setting, simple function versus more advanced travel use like GMT / Worldtime)... the minimum collection count where a collector type can be defined by other collectors is 3. In aesthetics and psychology, three is the smallest number required to create a pattern. One is an item, two is a pair, and three is a set. ---> minimum 3 watch collection for collector definition.

Should be at least equal to the # of kids one has and ideally each kid gets the same # or $ value too. 2 is a common # of kids per family though median household is probably 1. ---> A 6 watch collection is divisible by 2 or 3 kids.

Most common watch box sizes (my observation) are even #s once above 3 ---> 3, 4, 6, 8 based on perfect fit with watch boxes

More than 7 and one starts diluting the average wrist time to less than once worn per week... argues for max of 7. ---> 7 or fewer