I'm going to assume you are referring to a Hoth scene and mixing AT-STs with AT-ATs to recreate that battle. If you're talking Endor, you just need one AT-AT with a shuttle and 2-3 AT-STs for the Jungle. For Hoth, I would include 3 AT-ATs and 2 AT-STs mixed together, provided you have the space. The formula for me is one less AT-ST than AT-ATs. If you just have space for one, I think they're cool on their own as well. Maybe just use one of each in a Hoth scene with Luke ascending on his grappling hook.
My vintage Hoth display has two Hasbro AT-ATs and one AT-ST. Would have included three if I had the space, but they're two crowded on a 2ft shelf. I also included two Imperial Attack Bases and a couple of tripod laser canons in mine.

On a side note, if anyone is looking to add some ice and snow to their display, I picked up a GOT Ice Wall playset on clearance @ TRU for $20 a few weeks back. They still have them at my local store and online. Kind of makes a nice backdrop of ice if you take the wood parts off. And you get a free action figure of the crabby writer from Elf too!
