A good estimate would be to go by shipping numbers per year. Apple are at over 12 million Macs per year currently. If you run it back to 2000 when OS X was introduced, that will give you a good estimate. Given that they are selling more than ever now, your upper bound will be 120 million i.e there are under 120 million Mac OS X users. This will soon be surpassed by iOS users if it hasn't already been.
Variables not taken into account would be people running OS X on pre-2000 hardware and people running hackintoshes but those numbers should be insignificant.