If you look at the scoring system and you are ranked 4000 in something you've done once (and sucked at), you can assume you're at best tied for 4000th with people who have never done that event, and therefore there are at least 4000 players. This doesn't take into consideration people with multiple accounts, comped accounts, or inactive accounts that might still be in the scoring system somehow, so that number can't be validated without info we don't (and shouldn't) have.
For example, my only (and last) sortie in tour 34 was one fighter sortie with one assist and a high-altitude ack death at the end of a 12 minute sortie. This earned me a fighter rank of about 4000 an overall rank of around 4400, indicating that there are at least 4400 active accounts, and at least 400 people somehow had a worse fighter score than my one prematurely-ended attempt. The actual number can be ANY number above this however, as anyone with the same or worse performance will be tied for the same low score. For example, since I had no bomber score, I was tied for approximately 2700th, meaning that just over 2700 players out of over 4400 bothered to even start a bomber-scored mission, since launching a bomber mission and getting shot down fairly quickly should give a somewhat higher score than not attempting one at all.
Crystal clear? Short version for the attention deficit crowd - look at the scores. There are at least as many accounts as the worst person's rank, and there are probably more because some accounts are completely inactive throughout an entire tour. How many accounts are paying accounts can't be determined from the scores however.
