The reading books and aligned phonics here is a guide.

Our reading books are carefully matched to the phonics scheme (Essential Letters and Sounds) to ensure they are fully decodable.

With regards to word count, the definitive book for Book Bands is Which Book and Why? Sue Bodman and Glen Franklin (Institute of Education Press).  The factor they list for banding a book are complexity of content and language structure, complexity of format, complexity of book structure, complexity of use of alphabetic code, and the changing role of illustrations.  Not simply word count.  Three sentences of James Joyce is going to be way harder to read than Green Eggs and Ham (which is 775 words and would thus place at Purple Banding, phase 6, despite only having 50 different words, only one of which has more than 1 syllable or 5 letters).

Reading Bands Comparisons - Phonics Phases/Year groups and reading scheme