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Two instances are enough for fail over as read replica in other AZ would become primary in case of fail over. Refer Document DB Failover.
Based on load, volume and additional factors such as SLAs, RTO, RPO, this can further be extended but primary purpose of HA would be achieved using two instances.
Should you have any additional questions, comment here.
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I have to use the regional cluster to remain on the same region zone (data locality requirement). The region has 3 availability zones. The main question is 2 instances enough for high availability or I need 3 (depends how quorum is calculated).
Thank you for clarifying the question. Two instances are enough for fail over as read replica in other AZ would become primary in case of fail over. Refer Document DB Failover. Should you have any additional questions, comment here, if this answers your question, please click "Accept answer" and upvote it.
PS: I'm updating my answer with these comments.