How does the pricing for the Google Maps API work?
Google gives every user $200 per month free for the usage of Google maps. It is billed per thousand views. For most businesses that do not get hundreds of thousands of views per month, you will never go over the free amount, however, you will need to input a valid billing method in order to be able to obtain an API key and receive the free $200 per month. You can review the pricing table here.
A single key can be used multiple times, in the case of multiple maps used on the same page it would only be necessary to add an API key for the first map.