Welcome to the MongoDB Community @Nikita_Mikhaylov !
At least 3 AZs (with 1 data-bearing member in each) are recommended for a replica set deployment if you want to allow automatic failover between multiple DCs. This is the standard deployment set up used by MongoDB Atlas for high availability:
- Highly available : A minimum of three data nodes per replica set are automatically deployed across availability zones (AWS), fault domains (Azure), or zones (GCP) for continuous application uptime in the event of outages and routine maintenance.
For more information on deployment considerations, please see Replica Set Deployment Architectures in the MongoDB server documentation.
How does the maximum number of voting members (7 in a replica set) limit your planned deployment?
An arbiter is only needed if you have an insufficient number of voting members to ensure a quorum to sustain or elect a primary, but I strongly recommend avoiding arbiters if possible. For more context, see Replica set with 3 DB Nodes and 1 Arbiter - #8 by Stennie_X.
Regards,
Stennie