I think it was 50 coins. You were allegedly supposed to just tear through the SMB and Rad Racer portions and rack up all the points in Tetris. But I remember reading somewhere that there was an easter egg of sorts where you could hang out in SMB, warp to one of the worlds, and rack up a ton of points there. The "triathalon" cart is allegedly available as a NES ROM image for emulators, but I have never bothered to try to track it down.