I'm surprised it hasn't been. Some initial reporting measured expansion of the camps by floor space which can be used to estimate internment numbers. One of the researcher is a licensed architect, this is 101 stuff in terms of occupancy/building use estimation. Source some commercial satellite service to take pictures and start counting.

That said, if the intention of the system is to indoctrinate then we can expect most of the 11M Uyghurs to be imprisoned at some time. Really the question is concurrent numbers which makes for grabbing headlines. My personal guess is if competent geospatial analysis was conducted the actual detention capacity would be far lower than the commonly cited 1-3 million Zenz estimated extrapolated from his claims of 1,200 detention centres. This study, alleged to be comprehensive, found 260 (edit 315) camps in the entire region. At least it's a good start.