There is currently no way to do this within Photo Mechanic. The best suggestion I have at the moment is to export the keywords to a TSV file using the Text Exporter, then use a formula in your spreadsheet software to calculate the number of keywords in the column where you put the keywords.
The following formula works to do that in MacOS Numbers (and likely Excel too)
=IF(A1="","",LEN(A1)-LEN(SUBSTITUTE(A1,",",""))+1)
Replace A1 with the column where the keywords are