Not only are Frost Caves semi-rare, they can be hard to find. I recently explored a large mountain biome, which was more like 2 combined and only found 2 Frost Caves. Later after taming some lox, I made a beeline for my mountain base portal and stumbled across a 3rd Frost Cave tucked behind some rocks in an area I thought I had previously thoroughly explored. As Damnion said, "Easiest way to find large mountain ranges is to take out a ship and look for them from the water." Most of the ones I found doing this did have a lot of plains around them, but almost all them also had one side where the mountains had a steep slope access right along the water's edge. Frost caves are generally always found along the slopes at the altitude where obsidian first appears.