Bravo just confirmed a few things about Halo 5’s population on Reddit:
“I do believe you’d be very pleasantly surprised with not only the number of players who jumped on during the free play period (holy moly lots! by golly it worked!), but also the amount of Halo 5 players each day and month - this isn’t something we’ve talked about yet, but I am rather happy to share that even before the free play period, Halo 5 has had the highest monthly active players for a Halo title since Halo 3.
Also exciting (and not yet discussed, either) is the increase that H5 has seen in players each month over the past few months (we just got done looking at June numbers, which are once again higher than the previous month). As you can imagine this is pretty darn exciting to see, and this thread seemed like an appropriate place to share this news for the first time 
I’d have to get approval from the powers that be to say more in terms of specifics, but you should rest assured that together, we’re quite a strong army.”
This just goes to show, in my opinion, what our fanbase really wants: a Halo game. Halo 5 is the most Halo-esque title since 3 in terms of gameplay, and its monthly active players is now also the highest since 3. I personally think this is well deserved- the multiplayer ( and for sure the game itself) has had its fair share of problems (playlists etc) since launch but it still put Halo on the right track- going back to what we loved about it in the first place.
TL;DR, population is good, because the game is good. Let’s keep that trend going, 343