When can we expect a maximum number of SQL connections for the Central Administration Configuration & Content DB? and what are all the important tables in these databases that an administrator should know ?
You can expect a lot of connections to the SQL Servers when you have a lot of requests and when you are using code that is not properly managing and disposing the SharePoint objects.
Since you should not fiddle with the databases at all you should not recommend any tables to your DBAs - then they would start "fixing" indexes and stuff and potentially harm the SharePoint databases. Leave them as is and instead focus on the databases;
- which DBs are used for collaboration sites: optimize for read/write
- which DBs are used for anonymous publishing sites: optmizie mostly for read
- which DBs are used for records management sites: optimize for read