Note showing total number of parts

I have an assembly with a part that is patterned and then that pattern is patterned.  I am trying to make a note in the drawing that will link to the total number of this part in the assembly.  I know how to link a note to a custom property, but I cannot figure out how to create a custome property that is the total number of a certain part in an assembly.  There must be some way to click in a Custome Property field and pick a variable and multiply it or something.  Would an equation work? Any ideas?  Thanks.

SolidworksAssemblies