Most of TF2's player base are bots used for either farming or trading or cheaters idling alts to make their accounts look more legit. We've known about this for awhile because there are a couple of massive refined bot farms ran by a person or two and when they turn off the player count sharply drops.
This website tracks the number of players in any server in TF2 world wide.
As of this post Steam is reporting 48,966 players are playing Team Fortress 2.
As of this post that website reports only 6,401 actual players currently in any servers.
Part of why this discrepancy can happen because Steam's API can be told you are in a game even if you do not actually have it open. Idling programs do this (you'll see trading card AFK farm programs do it for example) and it's actually dead simple to do this.
That means 42,565 players are unaccounted for and are:
- alt accounts used to refined metal bot farm
- bot accounts used for automated trading & inventory collection/management (auto accepting items into inventory, auto crafting to make space, etc)
- cheater alts idling for hours to look more legit/cheat in competitive PUG groups that require certain amounts of hours like TF2Center
- people who are legit just idle on the title screen (obviously this is a small percentage though)