Do you think you should be DBA for more than 1 DB engine like (Oracle , Sql server , DB2 ...) or it's better to focus on just one DB engine?

DBA should know about multiple database engines, as database administration work may require transferring between different platforms.

its good to have more than one knowledge in database, as for me am good in Oracle, Hana and Sql server Database

more than one mysql ,mssql 

Specialist in one DB engine

Specialist in one but Expert knowledge in Multiple DBMS

It's good to have knowledge on multiple database. I'm good at SQL server DBA , Dynamo DB and Mongodb

More Than One DB (Oracle, HANA , SQL server