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It kind of depends on your goals and purpose of these 20 distinct domains as well as your current management state. Assuming a lot of things not in evidence here, you can sync your multiple on-prem AD domains successfully to a single Entra tenant which Intune will be linked to. The devices from these domains can then be either hybrid joined to this tenant or reprovisioned to be Entra joined to this tenant. In both cases, this will enable the devices to be managed by Intune.