[DrShadow]

I'm using Allen 4 bike rack for 2'' receiver (https://www.amazon.ca/gp/product/B0792V7HB5) with oem trailer hitch. Fills very strong. Good space for 2 bikes what I'm usually use it for. It will work for 3 bikes , but I have doubts it will have enough space for 4 full size bikes.
Reason of using 4 bikes rack - it more strong than 2 bikes one and as result you are getting almost no wobbling or shaking with 2 bikes on it.

 

[DrShadow]

If you have basic aluminum road bikes you are probably okay. If you have carbon with irregular shaped tubes you may not be able to use that style. I believe hanging bikes are more prone to sway as well. If you don't want your bikes to hit each other, that may not work for you. Dr. Shadow may have more input.
You haven't said what you are carrying so I thought it beneficial to offer a though in case you aren't aware.

You are right - if shape of bike frame irregular may be adapter will be needed. One of my bikes like this (2022 Supercycle Radex 26, Dual Suspension Mountain bi...) . It still fit , but not hangs exactly horizontally.
I'm using adapter (https://www.amazon.ca/gp/product/B0817M1S8Y) mostly because it's easy to hang bike in garage and on the rack. Adapter will work for any crazy style frame bike and Allen rack.
Also rack is 2'' and has no problem with 3 heavy bikes, they have 1 1/4" mount as well but it looks weak. Also I'm using ratchet strap through bottom of bikes frames and hitch for additional security.