What is max no. of database we can have in a single instance of mongodb
Welcome to the MongoDB community @Avanish_Gupta!
The maximum number of databases and collections is a practical limit determined by a combination of factors including your system resources, schema design, workload, and performance expectations. If your working set is significantly larger than available RAM, performance will suffer and eventually become I/O bound shuffling data to and from disk. You can scale a single server vertically (adding more RAM and disk), but it will eventually be more economical to scale horizontally (across multiple servers) using sharding.
For more details, please see my response on Database and collection limitations.
Do you have a specific concern around an application or deployment you are designing? If you can share more details around your use case and concerns on the number of databases, there may be more relevant advice to share.
Note: I generally only recommend using a standalone instance for development or testing purposes. A replica set provides data redundancy, failover, and high availability features which are typical requirements for production deployments.