A pair will probably be your best bet then
It doesn't hurt to get a big enough cage for larger groups in advance though,since you could potentially reuse it over and over for different groups of rats over time instead of having to buy all different cages depending on the size of a group!
By the way you can turn grape vine into bird or rat perches/climbing branches if you have access to even the most basic of drills and the right stainless steel hardware.It isn't difficult either,and can be much better to get branches that you really enjoy rather than being quite as limited with just the premade bird perches.It works with any sort of safe branch,although grape vine is definitely a good example!