I have a Twinny Ergo tilting rack that sits on the tow bar. You need to be aware that some of the cheaper towbar mounted racks need a bracket bolted to the towbar mounting, so will not work with a removable towbar. Also, most racks clamp onto a stadard bike frame, so cannot accommodate odd bike designs. Some of the Twinny designs grip the pedal crank so can take any bike design. Twinny make ones for more than 2 bikes. Some can be extended as required.
Avoid any of the cheap stuff that attaches to the tailgate with straps. The German RAC did an emergency brake and lane change test with bike carriers. Every single one of ones held on with straps came off the car completely during the test.