Ideally it would have two antennas.
Amazingly enough, the reason is that at times (more in the past), you could fly over the top of an ATC antenna and be using your TOP SIDE antenna and the wings/fuselage could block the signal to the ground.
So, you switched to the OTHER radio which had the antenna on the bottom and would have a good signal to the ground.
I can't tell you for your plane, but count the number of com antennas!