Learn Italian: Usano molte più espressioni regionali di quello che pensavo. - They use many more regional expressions than I thought.

When you compare a noun to a whole conjugated verb clause (like 'I thought'), you can't just use di or che. You need a phrase like di quello che (literally 'than that which') to connect the comparison to the action.

Could I use di quanto instead of di quello che in this sentence?

Yes, but be careful with the verb! Di quello che is typically followed by the standard indicative mood, like the imperfect pensavo. If you use di quanto, it requires the subjunctive mood, so the sentence would become di quanto pensassi. Both mean exactly the same thing.

Why is it molte più instead of molto più?

In this sentence, molte (many) is an adjective directly describing the noun espressioni (expressions), which is feminine plural. When molto modifies a noun, it must match its gender and number. If it were modifying an adjective or verb (meaning 'very' or 'a lot'), it would stay as the fixed word molto.

Why use the imperfect tense pensavo instead of the passato prossimo ho pensato?

The imperfect pensavo is used because your thought was an ongoing, continuous belief that you held in the background over a period of time, right up until you realized how many regional expressions they actually use.