My understanding is that if you have any of the enterprise licenses mentioned, you are fully covered for MDI (no limit on activity though I think there is a platform limit of 350 DCs). I also assume standalone is based on the number of domain controllers. MDI runs on DCs in one or more forest. No additional charges based on usage. Most inherit licensing through EMSE5.
https://learn.microsoft.com/en-us/defender-for-identity/prerequisites#licensing