This is managed by the driver. Usually, there is an option to close connections.
If you’re seeing a spike or a huge number of connections open please make sure the application is not creating lots of connection pools and never releasing it.
This link for java driver might help:
Please notice: [h=5]IMPORTANT
When creating many MongoClient instances:[/h]
All resource usage limits (max connections, etc) apply per MongoClient instance
To dispose of an instance, make sure you call MongoClient.close() to clean up resources