I wanted to publish my book in 5 parts (not parts like sequels, but 5 parts, 20-25 thousand words each launched month after month) Do I need 5 different ISBN numbers to it, or just one will be fine?
Each book will have its own ISBN. If it is later sold in a boxed set, or as a bind-up it will have a separate ISBN for the whole thing.
Keyvee -- Note that this holds true if you're talking about five e-books, as well. (And if they are both physical and ebooks, then you will need different ISBNs for the ebook and physical-book versions)
All in All, if I want to divide my book into 5 different parts, I need to have 5 different ISBNs. More over, for those who wish to order the 5 parts as one whole book, I need another, sixth ISBN number.
Correct. And by the way, the N in ISBN stands for number, so you don't need to say 'ISBN number' because you are saying 'international standard book number number'