How can I determine M20K number from block memory bits value?

You will need to run until fitter finished in order to see this report. You can also estimate the M20k number by looking into the synthesis report of the block memory bits / 20. You may refer to https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/hb/arria-v/av_5v2.pdf table 2-1 as an example.