Mario was in 12 games starring him in 21 years.
You are really, really leaving out a bunch of titles if you only come up with 12....
Super Mario Bros
Super Mario Bros 2
Super Mario Bros 3
Super Mario Land
Super Mario Land 2
Super Mario World
Super Mario World 2
Super Mario 64
Super Mario Sunshine
New Super Mario Bros
New Super Mario Bros Wii
Super Mario Galaxy
Super Mario Galaxy 2
These are JUST the sidescroller / platformers that star Mario. You can also include the following platformers, although the first 2 arcade games lack side scrolling and the others add RPG elements, but all of these star Mario:
Donkey Kong
Mario Bros
Super Mario RPG
Paper Mario
Super Paper Mario
We're up to 18 now. Other games that star / costar Mario, but are of various other genres:
Dr Mario
Mario Puzzle Party
Mario vs Donkey Kong: Mini-Land Mayhem
Mario & Luigi: Partners in Time
Mario & Luigi: Bowser's Inside Tale
Now we're at 23... I can't think of any more that star Mario off the top of my head, but I know there were some Mario pinball games and a few other Mario puzzle games. And where do you even put Mario Paint?
Now, if you include other Nintendo platformers that don't STAR Mario, but do have platformer mechanics that are somewhat similar to the Super Mario series and star other prominent Mario universe characters, like Wario, Peach, Yoshi, and Donkey Kong you get:
Yoshi's Story
Yoshi's Island DS
Super Princess Peach
VB Wario Land
Wario Land 2
Wario Land 3
Wario World
Donkey Kong Country
Donkey Kong Country 2
Donkey Kong Country 3
Donkey Kong Country 64
Donkey Kong Country Returns
These are just the sidescroller / platformers based on Mario and his ilk. I'm sure I'm missing at least a few in there as well. I'm not going to count all the puzzle games, board games, mini game collections, racing games, sports games, fighting games, and other misc action games that don't star Mario per say, but are based on the characters in the Mario universe.
This is JUST ONE of Nintendo's franchises. You can complain about COD, Halo, and even FF until the cows come home, but the fact is, nobody, and I mean NOBODY, milks their franchises the way Nintendo does.