Using the term "#1" is too abstract. You have to look at in football speak. In an offense like the Hawks run, you have the X, Y and Z receivers (or split end, slot and flanker).
Rice is a traditional X receiver (split end), Tate is a traditional Y receiver (slot)..........so that leaves a gaping hole in the Z spot. So when people say we need a #1 receiver, they mean we need a good Z receiver, which traditionally lines up behind the line to avoid jamming, has blazing speed and can stretch the defense.