Google Maps API key for network

We plan to include Google Maps as a Pro Sites feature. We are also using Blog Templates. Does the Google API need to be site specific or can we create a network API. I see that you do not have network settings but I am wondering if we put a "network" api for the main site in the blog template if this would work or do we need to instruct our subsites to get their own API? We are always looking for ways to simplify settings for our subsite users.