I’m new to the hobby and trying to plan out locations of pi and transmitter. I’ve heard if they are too close together you can get interference. How far apart should they be?
Well, that is a tricky question. There are some that can run them real close and some that have to run them far apart. Some of the main things are the use of quality power supplies and cables between the two. Some have found the need to use a ferrite isolator on the cables. I think it also comes down to the type of Pi being used and the model of transmitter also.
Having said all that, I normally have about 3 or so feet between my PI and transmitter while using a quality splitter to go from my PI to my external speaker and to the transmitter. The best thing is tog et your setup done early and tested to see what works for your situation.
But this is just my way of saying that I don't really know... Try doing what you want, and if that doesn't work out, apply the various workarounds (increase distance, add audio/power isolators, etc.).