I'd go with 10x census. But there has to be a lot of attrition due to mom throwing them out, water damage, young family members ruining them, critter damage, fire damage. Plus even if there were 200k printed how many did not sell off the stands and were remaindered and or then destroyed.
I'd vote for more than you think are still out there, but at the same time a lot less than a maximum number printed. Plus it was a new charactor but it was not in a #1 issue form. So I would bet Spidey #1s out number AF 15s by 5 to 1 or maybe even 20 to 1.