MapTiler Cloud billing model is based on the number of requests, so you pay only for what you use. A single map view usually generates 4 requests if vector tiles are used, or 10-16 requests for raster tiles with 256px size. See the requests counting demo here.
Requests counting table
|TileJSONs, Style JSONs, Fonts, Viewers, XMLs||Free|
|Rendered raster 512x512px tile including HiDPI/retina||4|
|Rendered raster 256x256px tile including HiDPI/retina||1|
|Single tile served from .mbtiles||1|
|Static maps API image||15|
|Vector data (GeoJSON)||1|