AFAIK, it depends on TDP. So, yea, you won't get all of them running at max turbo. (Even if you did, they could still be running at a lower p-state and just reporting a higher frequency. Frequency and performance aren't really the same thing anymore)

The Boost-ed frequency depends on multiple factors, including the number of cores used, power consumption, current, temperature and even the type of instructions executed (e.g. AVX instructions trigger a drop in frequency on Xeon cores, not sure about Xeon D, though).

The boost levels are typically defined as "bins", i.e. the max clock if 1, 2, 3, etc. cores are active. Can't find Xeon-specific info, but this page shows some general information on the topic: https://www-ssl.intel.com/content/www/us/en/support/processo...