if Onyxia merge with Icecrown

how i will be able to keep all my characters from both realms on single realm? Character list will be expanded or i will lose characters from Onyxia?

you can use barracks to disable some characters and free some space for the new ones if you wish.
https://www.warmane.com/account/barracks
(you have to manually transfer the characters from onyxia to icecrown , so no they don't get deleted if they can't be moved)
you have some free time to take that action by yourself, i don't remember how long.

Once the transfers are available at the end of the WotLK, you'll need to start the transfer process through https://www.warmane.com/account/services.
Select character(s) you want to transfer to begin the process. The process may take a while.
Once the process is done, your character(s) will appear on Icecrown's character selection screen. If you go past 10 active characters, the further characters transferred from Onyxia will be moved to Barracks for storing purposes.
The offered character transfer periods have usually been around 1 year. So, you're not in a hurry, but don't forget to do it if you value your characters and want to continue playing on them.