A map unit is defined a as a single 256x256 raster map tile with a single layer. Whether you're using interactive map tiles or a static raster map, the map units will be calculated the same. Please keep in mind multiple layers means there are additional multipliers to consider. For example, let's say your application calls for a 900x600 static map with the flat-dk base layer, satellite and radar weather layers, and an admin-dk overlay layer. The request would look something like this:
https://maps.aerisapi.com/[client_id]_[client_secret]/flat-dk,satellite,radar,admin-dk/900x600/39.7,-93.83,4/0.png
Let's start by figuring out how many tiles make up one layer:
4 columns * 3 rows = 12 tiles for 1 layer. As we are requesting 4 different layers (flat-dk,satellite,radar,admin-dk) we take 12 * 4 which is 48 total map units for the example listed above.