Check out the Stovebolt.com Gallery. 3600s were less common than 3100s, but would most likely not be considered to be rare. In rank order, 3100s were most common, then 2-ton trucks, then 3/4 ton trucks. 3/4 ton trucks were about twice as common as 1 ton trucks.
A very crude estimate can be made for numbers of 3100 and 3600 Advance-Design trucks produced from 47-54, but there would be no good way to speculate how many are still running.
Total Advance-Design Chevrolet truck production by year is a available. GM documents giving total production (and % production) for each model are available for a few years. One could use those % for all years and arrive at a crude estimate of 3100 and 3600 USA production.
3100 trucks were about 54% of the A-D trucks built (over 1.2 million A-D 3100s)
3600 trucks were about 14% of the A-D trucks built (over 170 thousand A-D 3600s)
