I work there. They've expanded a lot. If my reckoning is right Google occupies 8/15 floors completely, and occupy fractions of a couple more floors.

As of last year (when I worked there), 8 was a pass-through floor mostly occupied by ISPs. 15/9/10 should be done by now, though.

Is the rest of it empty or do they rent it?

Their tenants include Nike, Deutsch Inc, Bank of New York and Barnes & Noble. They also donated space for Cornell NYC Tech to hold classes there.

https://en.wikipedia.org/wiki/111_Eighth_Avenue#Google

Oh man. One of my favorite memories was the look on the Internap security guy's face when we parked a 19ft box truck at their loading dock on the 10th floor of the Port Authority building. "the truck is WHERE?"