so 500 MTons is about 500x100 people - 50K people.
But for large scale, the scaling isn't linear, and peak production is going to be closer to 6x18x52 or 5.6MTons per 200 people, a factor of x2.3...
... so it could be as low as 21K people. Assuming TL7-8 mining.
Given my other assumptions, handwaves, and ignored but important details, I would be happy with any number between 10,000 and 100,000 people directly involved in the salt mining process.