Layflat books now have a maximum of 140 pages across all formats. (Beyond this amount, they become too thick for the binding equipment to handle.)
Please also note that the more pages, the less they are able to lay absolutely flat. A 140 page layflat book will exhibit slight curvature in the binding zone. However they still lie reasonably flat (without needing to hold the pages open) and are far superior to standard paper books of the same page count.