Some one told me " if i want to make a big sale, you must make a Mac Version" Does he mean it is easier to sell on Mac than Win?

Your friend was probably referring to the fact that there are fewer high quality games available on the Mac than on the PC. Medium quality games tend to sell better on Apple products because of the scarcity.

That should not be your only motivation for picking a platform to program on. With Apple moving away from the PowerPC and toward intel hardware, they've opened themselves up to the mainstream *nix games that contain x86 assembly code, in addition to all the portable unix games out there.

Develop what you enjoy on the platforms available to you.

I also think it is referal to the success of games like WoW, because it runs on all platforms without
modification. And games that are crossplatform can get a much larger market from gamers who wouldnt otherwise play.
Expecially if you are targeting moderate hardware instead of top-of-the-line gamming PC's.

I looked into the popularity of platforms for indie games myself a while back. I'm afraid I can only post anecdotal evidence as I don't sell games and must rely on information provided by other people, but the impression I got was that, for indie games especially, the Mac OS version would sell in significantly higher proportions. I remember reading figures for one game that published for Windows, Mac OS X and Linux, where about 80% of the sales were for Mac (15% for Windows, 5% Linux). Sadly I can't remember the game's name off the top of my head.

This was before the Macs were released with Intel chipsets making them easier to run Windows, so I don't know if things have changed. However for indies I'd still think it's best to release both a Mac OS and Windows version if you can. As so many games are strictly Windows only it's worthwhile to aim cross-platform to reach the lesser-tapped markets who'd love to buy your game for their platform.