not many. when they are small you can keep them in a 30 for a awhile, but as they get mature, you will have problems with the "pecking order" in a tank that size.
its not much room for the weaker fish to get away from the dominant ones.
a 55 is sort of the minium size for a decent discus tank for adults. you could keep 4 in a 55.
but a 30 is too small.
back in the day, I tried just two in a 30 and they were fine until they got about 4-5 inches. Then they started to fight..
I would reconsider getting discus until you get a larger tank.