Imagine if this forum had KYC required to create your account. That 3.5 million registration will be impossible to achieve. I would say umm almost 1 million accounts are original. Rest 2.5 million are multi-accounts of those 1 million.
First of all, if this forum ever had KYC, it would be already dead.
Regarding your estimate, I think that its very optimistic to think that 1 million different people actually registered account here. Based from what I've seen around here, my guesstimate is that number of alts are in the region of 90%.