Hi @Dimitri Goossens ,
To the best of my knowledge, there's NO difference in the amount of ActiveSync devices a user can have between the Standard or the Enterprise CAL. The difference between STD and ENT CAL mainly lies in the supported features. For the detailed feature breakdown for each CAL, we can refer to the table in this link.
Regarding your concern about the amount of ActiveSync devices, as in Exchange 2016 you can still license with either per-user or per-device CALs, so personally I'd suggest comparing them and choosing a more suitable type for your environment. For example, If several users share a desktop and do not individually access the Exchange server from other locations, such as their home PCs, then they can be licensed with one device CAL. If a user needs to access Exchange from multiple locations, then a per-user license would be preferred.
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