The head of a pin 1.5mm in diameter has a diameter of 1500 microns. Pi R-squared makes the area of the head of the pin equal to 1767145.867644 microns.

The largest colloidal particle would be about .01 microns in diameter. Pi R-squared is pi times .005 or .015707996326795, the area that the particle would occupy. Thus the number of colloidal particles .01 microns in diameter that can fit on the head of a pin that size would roughly be 1767145.867644 divided by .015707996326795, or about 112,500,000.

Obviously that is very rough because the particles aren't perfect circles, but how's my math on that?