SInce no one seems inclined to answer your question, I will. However, I
would recommend that you find this type of answer in a textbook as it
should be very easy to find.
a mol is equivalent to 6.02E23 molecules.
therefore a mmol is equivalent to 6.02E20 molecules.
it refers to an absolute number of molecules.
on the other hand, M corresponds to a concentration. That is 6.02E23
molecules per liter.
therefore a mM concentration is equivalent to 6.02E20 molecules per liter.
hope this helps.
Martin Houle M.Sc.
Centre de Recherche du CHUL
Laval University, Quebec, Canada