isn't lui ha, the same as just ha? :/
Generally yes, but in a question you should write "Quanti libri ha lui ?" or "Lui quanti libri ha ?".
Why you don't use the pronoun "lui". A man and a woman can be sitting together and each one have several books, you have to clarify if you want to know the quantity of the woman's books or the quantity of the man's books .
Why "Quanti libri lui ha?" is not correct? ;(
The subject goes at the end of every question in Italian, it seems.
If quanti is the correct adverb, then why does the word definitions say come molti?
why can you not use 'hai'? thought this was the informal 'he has/she has.
Hai is you (singular) have