9 - Is this a test answer or do you just want to know for a project you're doing? Take a look at NEC chapter 9 - tables 4 and 5 (in the 2005 edition at least)
It varies by conduit type. If you don't know the type, use 9.
The sizing of conduit is based on conduit fill. Depending on how many #12 wires you want to go in the conduit will depend on what size conduit is required.
9
11
6 nches
10
ONE
30 pairs
14
9-10, but the conduit may not exceed a certain length, or a certain voltage in order for you to "legally" do it. Many applications allow for conduit fill ratio's that exceed the standards of the NEC ( National Electrical Code) and most of them are short (less than 24") "chases" between control devices mounted in listed Junction boxes for that purpose. It is not advisable to fill a conduit more than the NEC allows for purposes of heat dissipation.
4
Two quarters of an inch are in a half inch.
6
According to the 2005 NEC Annex C, table C.1 you are allowed nine #12 THHN conductors in a 1/2" trade size EMT conduit. If your conduit run is any length at all, you would be wise to increase the conduit size to 3/4". as pulling 9 #12's for any length in 1/2" EMT is very difficult.