For quantification of molecules on the surface of cells you need to measure reference particles with a defined number of molecules bound, which in turn will allow you to calculate back to your molecule of interest. The correct terminology is MESF (which stands for Molecules of Equivalent Soluble Fluorochrome)

There are various protocols around how to do this, here is a link to one:

https://www.bangslabs.com/sites/default/files/imce/docs/SS%20Quantitative%20Cytometry%20Web.pdf

Some more background information on the website of Purdue:

http://www.cyto.purdue.edu/cdroms/cyto6/content/general/quantit.htm

Good luck with your measurements!