DBMS Classes In Symbian

What is the main difference between RDbNamedDatabase and RDbStoreDatabase classes in Symbian

RDbNamedDatabase provides shared access to the data in the tables.
Use it when data intense applications and multiple applications need data access.

RDbStoreDatabase provides a single thread access, in the sense only that application access from the thread.
Use it for simple data based applications

Why exactly two different classes provided?

From the beginning of database designs shared and exclusive accesses are treated separately. They are also divided depending on the intense of usage.

Is there any benifit for RDbStoreDatabase over RDbNamedDatabase class?

It depends on the thumb rule : "how u want it". If u need for a simple mapping purpose, RDbStoreDatabase serves ur purpose. If intense data transactions are required by multiple clients RDbNamedDatabase will help.
also when a file-store database is constructed using the RDbNamedDatabase interface, there is no access to the underlying store for compression.

and for ur question regarding lmitations on db plz refer SDK
Symbian OS v9.2 » Symbian OS guide » System libraries » Using DBMS » Interface to DBMS databases » Implementation limits
Hope it helps u.