I think this is really what most people talk about when they say they miss the WCHA; basically the 80s/early 90s. After Michigan St, Notre Dame, and Michigan left in 81 but before St. Cloud, Bemidji, Minnesota St, and Omaha joined.
WCHA was a very unstable conf save for that 80s and early 90s period. Even then, both Northern Michigan and Michigan Tech spent big chunks in the WCHA.
And really, its the core of Wisconsin, Rodentsota, Sioux, Denver, and UMD that people are talking about they talk about the WCHA. So 5 teams.