Google started in 2012 with the limit restrictions of the use of Google maps API.

The limits are up to 25,000 map loads per day for each API, so 25.000 for version 2 and 25.000 for version 3 (see What usage limits apply to the Maps API?). Google will only enforce the usage limits on sites that exceed them for 90 consecutive days.
To see if you reach the limits, you must take same steps to get an insight (see below).

It's not counted for each page, but every map on the page is counted.
Not clear yet is if a map is counted when it's not shown on the page for example in a lightbox, slider, tabs etc., because the plugin checks if a map is visible before loading the google map.
The geocoding of addresses on the server or in the browser have other limits.

Google Maps API version 3 doesn't require a key.

How google will restrict an application or website to load map if it reached it's limit is not clear at the moment.

If you want to have an insight in how many time a map is loaded on your website then you must use a Google API Key (see Google API Console). This key can be used for Google maps API version 2 and 3.
You can enter this key inside the plugin parameter at google API key or Googlemaps Multi API Key.

You can make different keys for different website. Restricts a key to your website(s), to prevent use on unauthorized websites. I've tested this restriction and it is not working yet.

 

In the Google API console you can see the usage of the Google maps API version 2 and version 3, see:

 


 

 

More info: