If you create games, you are presumably wanting someone out there to play it (though not always! Writing for yourself or your own gaming group is absolutely a thing as well)). However it can be pretty hard to actually figure out how big a game is. 

Obviously one answer is social media buzz but that's kind of hard to gauge. Warhammer 40K is obviously the biggest miniatures game in the market, but a large portion of the people talking about it online are not actually people who play the game at all. And for a smaller game, the social media buzz is likely to be a few hardcore folks on scattered forums rather than social media where all but the mainstream struggle to be noticed. ( This is all greatly oversimplified, please bear with me )

If the game is commercial, you have sales figures and I wanted to talk about that a little bit today. 

Fivecore 3rd edition has sold around 1400 copies in its lifetime. I don't remember how many copies were also "sold" when we were in the Bundle of Holding but I want to say another 1000 copies or so. Let's say it is 2500 all in all to account for give aways and other stuff I am forgetting.

So the easy answer is that there are 2500 players of Fivecore 3rd edition. But that isn't really the case:

Some people buy the game but don't end up playing it (they didn't like it, they never got around to painting that army, they bought it just to read for ideas etc.)

Some people did play but have moved on and won't return (do they count as players? Maybe not?)

Some are not playing currently but might in the future (do they count? Probably?)

Some are planning to play but have not yet (do they count? Probably?)

Some people pirated the game and while I don't think those people generally play games, a few no doubt do. 

Some people did not buy the game but play with their friend who does.

Some people play in a gaming group or club where 1 or 2 guys bought the rules but 4 or 5 play. 

Can you quantify all of these? Probably not. If I had to guess, I'd say two thirds of the copies sold is not a TERRIBLE estimate for a newer title and probably closer to half for an older title. It is entirely possible it is less than that. 

the Five Parsecs Facebook group has 4300 members and last I heard sales were somewhere above 10.000 copies, so that checks out if we assume that the people who sign up on facebook are more likely to play the game and that some amount of players never sign up for anything. (supposedly for video games 90% of players never interact with the game community in any form, I assume tabletop players are a bit more engaged but I could be wrong). 

Share your thoughts down below.