In Steam achievements it always gives:
% of all players
Is there anyway to know total number of players though?
There's no native way to do it*, but you can do certain math calculations with the information shown on SteamDB.
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.