Learn French: Il y a tellement de provisions que nous avons de quoi régaler nos invités. - There are so many provisions that we have enough to treat our guests.

The phrase avoir de quoi followed by an infinitive like régaler means to have the means, resources, or enough of something to do an action. Literally, it translates to 'to have of what.' Here, it implies having the necessary food to treat the guests.

Could I say nous avons assez pour régaler instead of nous avons de quoi régaler?

Yes, you could. Nous avons assez pour is grammatically correct and means the same thing. However, avoir de quoi is a very natural, idiomatic way to express having the wherewithal or the necessary materials to accomplish something, and it is widely used by native speakers.

Why do we use tellement de instead of just tellement or si?

Because it modifies a noun (provisions). To say 'so many' or 'so much' before a noun, you must use tellement de (or tant de). You would use si or tellement without de if you were modifying an adjective or an adverb, like saying someone is 'so tall.'

I usually see régaler used as a reflexive verb. How does it work here?

You are likely thinking of se régaler, which means to treat oneself or to really enjoy a meal. In this sentence, the verb is transitive, meaning the action is being done to someone else. Régaler nos invités simply means to treat our guests or to provide them with a feast.