Now I received feedback from Google, that confirms 28,000 MagicMirror map loads each month based on the $200 credit:
"Thank you for contacting Google Maps Platform Technical Support. My name is Xxx and I will gladly help you with your inquiry today.
Now, if you are using it to call Dynamic Maps, those 28,000 map loads are included within the $200 USD free monthly credit. Or, in other words, the $200 USD credit pays for the 28,000 map loads, ie, you get up to 28,000 map loads, for all intents and purposes, for free every month.
If you exceed the $200 threshold, then yes, your account will be charged according to the pricing sheet you will find at https://cloud.google.com/maps-platform/pricing/sheet/
That means that the monthly volume range above the 28,000 free map loads is 0.007 USD per each map load (or USD7 per 1000 map loads) up to 100,000 map loads.
If you want to make sure that you do not exceed the free monthly $200 threshold, you can cap your API usage  to a certain number of calls per day and/or set budget alerts .
We also provide an online calculator to estimate your usage at https://mapsplatformtransition.withgoogle.com/calculator.
I hope you find this information useful, and if you need further assistance or clarification on the documentation provided, please do not hesitate to let me know. I am happy to help!