Thank you for reaching out to our Geotab Community Posts! If I understand you correctly, you are wanting to know if there is a maximum to how many map overlays a custom map within MyGeotab can have. If you are stating that, there is no limit on how many layers an external custom map could have.
However, if you are referring to adding custom layers on top of our default maps (Google, OSM, Here, etc) then that is not possible.
Hope this helps!