Do I need an API key?
Answer: in the early days of google maps you did not require an API key (it is still possible to not have one due to backwards compatibility) however, today it is recommended that you generate an API key for google maps V3. Also, have in mind that some features are not available without the API key.
Generate your google maps API key HERE.
If you want to inform your self more about the API key, here is the official page that will tell you how to get started with it and how to include it to your project,
What are the usage limits?
Answer: If your site gets 25 000 map loads or more every single day, for more than 90 days in a row, the google team will get in touch with you (they are well aware of your google maps usage). If however you don’t think your google maps will generate such an amount of traffic then do the following:
- Modify your application so that your usage is less than 25 000 map loads per day.
- Enroll for automated billing of excess map loads in the Google Developers Console.
- Purchase a Google Maps API for Work license.
Here is the official page for the limits,
To sum it all up: If you can predict huge traffic on your website (and using the google maps API within your website), then once again read carefully the docs and see what further steps need to be taken, otherwise you can experiment with google maps as much as you want.