Most of these have their own subdivisions, like Producer, Director, Animator, etc., but it would take too long to divide them all out. Some of these might just be whole companies, some are pretty high, but others are quite low. I dunno.
Shirogumi Inc. - 73
D.A.G. Inc. - 27
TMS Jinni's Inc. - 8
Zinou Pharmaceutics, Inc. - 15
Agni-Flare Co., Ltd. - 16
Speed Inc. - 3
Tuetue Kombinato Co., Ltd. - 2
Daiuchu Kikaku Inc. - 3
Historia Inc. - 2
Katsugekiza Inc. - 27
Square Enix Co., Ltd. - 5
Echoes Co., Ltd. - 3
Crescent, Inc. - 1
Total: 185
These are ones under Special Thanks, not sure how some of them are involved or if it's just bad credits design.
Akatsuki Art - 1
Studio Hand - 2
unknownCASE Inc. - 6
D1 Inc. - 5
Streamline Studios - ~114
Access Games Inc. - 9
D4A Co., Ltd. - 7
APLUS Co., Ltd. - 2
charabans, Inc. 3
Sunny Days, Inc. - 1
G-STYLE Co., Ltd. - 4
PRIME WORKS Co., Ltd. - 2
BoobyTrap., Ltd. - 1
Torisan Co., Ltd. - 3
Virtuos, Ltd. - 28
New Generations Pictures, Inc. - 10
Music Productions (Not a company) - 18
Forcewick Sound Design Co., Ltd. - 3
Binari Sonori - 1
Testronic Laboratories, Ltd. - 11
Total: 231
This might be where the 12 number came from, there are 13 at the end with Directors and Producers and such.
For clarity's sake, my posts so far are not a grand total of everyone who was involved in the game. This was originally a question of who was directly involved in development.