We should advise decide where to deploy mongos and how many to deploy.

Customers are being advised in the mongos documentation:

The most common practice is to run mongos instances on the same systems as your application servers, but you can maintain mongos instances on the shards or on other dedicated resources.

However, if the customer has hundreds of application servers and/or are deploying mongos with their application server, we run into situations where there are too many mongos database servers running (and thereby causing problems) and/or there are thousands of documents in the config database related to mongos.