A quick look at the major cities in the area reveals a population of around... 130,000, but since there's also a lot of towns and villages in there... I'd guesstimate 200,000 souls at most in 2018, which would make it the least populated state in the Union by far. And based on what I did in terms of research on it, they were 1) banking on that sweet tourism money coming in from Mount Rushmore and Yellowstone, and 2) they hoped that they could present a united front against FDR's New Deal, which the people in those remote areas of their states were against, but their relative isolation and disunity meant that they didn't have the ability to "resist" that policy.
So the low population is kinda by design: all three areas are rural backwaters located in already rural backwaters.
Also here's something interesting I found on my hard drive.
Some proposed borders of Tibet. I can't remember where I found it, but I'm pretty sure it wasn't in this thread. Also while the map labels this as being from the Simla Conference, it's actually from the Simla Accord of 1914.