Anyway to know number of all players for Steam games?

There's no native way to do it*, but you can do certain math calculations with the information shown on SteamDB.

With Counter-Strike: Global Offensive and Shot With Their Pants Down as our example, we follow this steps:

Search the game on SteamDB

Select "Graphs & Spy" on the left side

We grab the number of owners (in this case, 39.980.725)

A calculation is done with the achievement percentage

0.786 * 39980725 = 31424849.85

An approximated of 31.424.850 players got the achievement.

Asterisk Note*: You can find extensions and user scripts for Firefox and Chrome, but that will require work setting everything up.

If you install the Enhanced Steam addon for Chrome/Firefox, it will show you player-statistics from steamspy.com on the game's Steam Store page