Can we calculate the atoms per cm^2 for silicon?

Well, you should study your subject before you take any public position. Or, as you will get told some day, "You should never say what you think without thinking." Silicon has a strictly defined crystal structure allowing you to count the number of atoms in any space you are able to measure accurately. Here is a link that explains how:
www1.columbia.edu/sec/itc/ee/test2/pdf%20files/silicon%20basics.pdf
If you want to know more, here are some more links:
www.google.com/search?q=silicon+crystal+forms