If you open the inspector on any of the examples you'll see how many tile fetches it triggers per second. So the only thing to worry about is how Google showed us couple of years ago how relentless it starts to charge for a previously free service - the Google Maps Javascript API - after it sees that enough developers got technically invested into Google Maps. The Map Tiles API documentation does not contain a 'Usage and Billing' section, which seems to imply that it is free to use, bound to the above mentioned limits.
Rate limit is 12,000 queries per minute for the tile renderer. This is calculated as the sum of all requests for all applications using the credentials of the same project. Maximum 250,000 renderer’s tile requests per day. Maximum 300 root tileset queries per day. Maximum 6,000 queries per day, calculated as the sum of all requests for all applications using the credentials of the same project.
