@Sarika , Yes, your understanding is correct Android Enterprise dedicated devices aren't supported by the Microsoft Tunnel.
https://learn.microsoft.com/en-us/mem/intune/protect/microsoft-tunnel-prerequisites#platforms
For your questions, here are my answers:
Q1. Can we use android fully managed enrollment approach by enrolling multiple devices with single user account (service account will be created at AD)
A1: In the above article, it mentioned that Android Enterprise Fully Managed support by the Microsoft Tunnel. You can create an AAD user and assign the Intune license to this user to do the enrollment.
Q2. Can this service account be used to enroll 300 devices with android fully managed enrollment approach?
A2: No, based as I know, single user can enroll up to 15 Android devices by using a single Intune license.
Q3: What license is needed for this service account?
A3: For the AAD account, we need to assign intune license. Here is a link with more details:
https://learn.microsoft.com/en-us/mem/intune/fundamentals/deployment-guide-enrollment
Hope it can help.
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