You can use as many as 8, but two would have to connect to the front inputs. Each input (coax 1 through 4 and optical 1 through 4) is completely independentl and can be connected to a different device. You can also assign two sources to the same digital input (such as setting up both CD and DVD to both use coax 1, a trick I use when playing CD's in a DVD player so that I don't have to change surround modes back and forth to have stereo with CD's and surround with DVD's).