Can I split off a 20a with a conduit body?
You said metal conduit--1/2" EMT &amp; rigid can have nine #12 thhn/thwn wires max fill per NEC annex C conduit fill. 

The area of the conduit body would only be needed only if a splice is made in that location, so the derate for more than 3 current-carrying conductors would be the next concern. 4-6 conductors require a 80% derating but the higher ampacity 90 degree table can be used for derating and you still meet code because the derated value is 24 amps. 

So this is safe for a 20 amp circuit(s).