Is a 3-bike rack needed for two bikes? I've been looking into getting a trunk-mounted bike rack for two bikes. However, I've heard a few people say that I actually need a 3-bike rack because there isn't really enough space on a 2-bike rack for two bikes.

Is this true? If so, why do 2-bike racks even exist? Does it just depend on the brand? 3-bike rack models are a bit more expensive than their 2-bike counterparts, and I don't plan on ever transporting 3 bikes, so I'd rather not buy a 3-bike rack if I don't have to.
It depends on how... scratch averse you are for your bikes.

A 4 bike trailer hitch rack by Thule can get 4 bikes on it. If you want to ensure that there is never any pedal to frame contact, then you can get 2 bikes on it.

In general with some wiggling and possibly removing a wheel or two, you can always get as many bikes as advertised on a given rack. Just be aware that there will be some rubbing as you fiddle with the bikes to get them on.