There is no minimum quantity when doing a digital run of books but you need to bear in mind that the set up costs of a single book will not necessarily be cheap. It depends what your budget is for a Christmas present. I always recommend a good price per book is around the $5 mark but obviously this is going to be impossible if you are only ordering one book. A good price for 1 softcover book is around the $50 mark and for a hardcover book around the $100 mark. Saying that 20 copies may only be $500-$1000 so the price comes down quite quickly. It depends what you are looking for.
As for how many pages for a hardcover book... you need at least 40 pages to make a perfect bound book (softcover book with spine on side no staples) and double that quantity is recommended for perfect binding in a hardcover as otherwise the pages won’t stay in the case. Saying this however there are always ways round this like making the pages twice as thick to allow you to have half the number of pages etc. Let me know how many pages you are looking at and I will make the best recommendations from there.
For more information on the dream book, check out 'How to Publish a Book for Pleasure, Profit or Publicity' THE SEMINAR or 'How to Publish a Book for Pleasure, Profit or Publicity' THE BOOK.