Depends on your definition of "community"!
I take "community" to mean anyone who actively seeks opportunities to play on a regular basis and would self-identify as an "enthusiast". (There are probably hundreds of thousands of folks who "like" pinball and would drop quarters upon seeing a machine somewhere, but would never entertain the notion of owning one themselves or of going out of their way to find them on location to play, but I wouldn't count them for the purposes of this thread.)
I think I remember recently hearing a podcaster mention having about 2000 regular listeners, and I'd guess that amounts to perhaps 10% of the "community" at best, so somewhere around 15-20k wouldn't seem too far off.
Could be absolutely dead wrong though!
(It's hard to extrapolate an estimate based upon Pinside participation, because my anecdotal experience is that there are a lot of pinball folks out there who do not post or lurk here at all.)