How many #12 thhn conductors are allowed in half inch emt?

2011 NEC- Annex C table C1 - 12 THHN conductors are allowed in a 1/2" EMT.

That does mean that the derating factors may not apply.

You really need to give us more information, except of course is this a schoolwork question. In that case, that's what your instructor is supposed to help you with.

Is the answer?
450
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350

I could only find 1/0??

How many #12awg current carrying conductors are allowed in a 4 square 1.5 inch deep box?

You found one answer to one of your questions. Continue reading the NEC and your study materials to find the other answer.

This is a valuable skill... that is, how to find the information that you need. The table of contents and the index are the best places to start... I'll bet most if not all of your questions can be found quickly in the index...
No one will ever "memorize" the code book, too much stuff that sometimes changes every three years. The more time you spend searching, the more familiar you will get with the structure of the code book...