Im currently in Chapter 5, all my characters are at level 12 and no one of them are dual classed. How many more levels can I gain up to the end of the game (normal)?

This my party:
Aasimar Paladin of Helm
Tiefling Fighter
Aasimar Druid
Aasimar Battleguard of Tempus
Drow Thief
Drow Evoker

I once thought of having a half-orc for a fighter/barbarian and a gnome for a mage.

Most parties top out around 15-17 (average 16) in normal play. That's with doing as many quests as you can manage to find as well as fighting monsters. So you've got about four levels to go.

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