What is the number of ground wires needed inside conduit with multiple circuits - same as number of circuits?

For example, if I run 2 or 3 separate 20A 12ga. Circuits inside 1/2 EMT conduit, must there be a 12 ga green ground conductor for each of the circuit pairs? Or can one common ground wire be used up to a JB where the multiple circuits split off.

For the specific case you describe, only (1) #12 ground conductor is needed...