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If you're just using base game content you're pretty much done - the classes.2da is already up to date to support 8 classes. In singleplayer you could edit ruleset.2da and put it in your override folder to enable 8 classes in singleplayer modules.
An example character is below. Some of the changes when you go beyond 3 classes include expanded spell books and feat menus.
NPCs with more than 3 classes
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