The subject, 'I', is understood, not written. The rest of the sentence (i.e. all of the sentence as written) is the predicate.
The verb is 'thank'. 'You' is the direct object. 'Very much' is an adverbial phrase modifying 'thank'. 'Sir' is a noun; its function is as a vocative, a term of direct address. It functions like an appositive in