What will happen for the database 1,2 and 4 on Instance A?
They'll carry on running on Instance A.
How many databases can be mirrored as per server?
Depends. Network bandwidth, server resources. I know there are recommendations, maybe try a google search.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability