I recently switched from Windows to Mac. Steam says I have fewer Mac specific games than total games and it should be an equal amount unless you can only download, install, and play once from your account. I thought I bought it so that it would work across all operating systems. If this is true, is there a work around or a discount for re-purchasing for the other OSes?
Once you buy a steam game, you can use it on any computer, provided that the computer can run it. There should be no need to re-buy the mac versions of the same games, though you will need to download the mac versions of the game to your new computer.
However, not all games for Windows work on Mac or Linux. That is why your mac specific games are less than your total number of games.
There are some workarounds involving getting windows to run on your mac (in virtual machines or something like wine). If you do that, you can install a windows version of steam and play your windows games there, but they are not mac native and may not work as well as you would expect.