Count duplicate rows

Hi,

How can i get all the duplicate rows to display as a single figure.

select b.book_title, b.isbn, pb.new_price, p.payment_amount, count(pb.quantity)
    from purchased_books as pb inner join book_info as b on b.book_id = pb.book_id
    inner join payment as p on p.payment_id = pb.payment_id
    inner join customer as c on c.customer_id = pb.customer_id



quantity should display 2 for isbn 1858818400 and 1 for isbn 067403063X


thanks in advance