Hi, Zeena, and welcome to WriteWords.
As it's non-fiction it's a difficult question to answer. There are recognised wordcounts for fiction, but with non-fiction it depends on several factors, including subject matter and the number of illustrations/photos (assuming you intend to include any).
Also with most non-ficion the writer would put together a book proposal and submit that to publishers who specialise in that type of non-fiction, and only go ahead and write the book if one of the publishers commissions it.
Probably the best thing to to find a few published books on a similar subject to yours to get a rough idea of the word count.