Steam's latest hardware survey, for March 2015, indicates that of 3.43% of Steam's 125 million users are using Macs. While that's a small percentage, that amounts to about 4.28 million people playing games through Mac on Steam.
The report also has a detailed breakdown of how gamers play on their Macs. The MacBook Pro line is popular, with 51.99% of Mac gamers using some version of that machine. It's followed by the iMac at 23.44%. The most common operating system is OS X Yosemite 10.10.2 at 50.87%, followed by OS X Mavericks 10.9.5 at 18.41%. The most popular Mac graphics processor is the Intel HD Graphics 4000.
Mac gaming on Steam grew over 0.20% since the last survey, though it's important to note that their numbers don't take into account other services. Many Mac gamers also utilize GOG.com and even the Mac App Store to find the games they love.
Source: Steam, via Mac Gamer HQ