Hey Chraz,
It's currently not possible for Left to give an estimate of number of pages as it does not have any sense of how long a page is, if you're in Japan like us, a page size is JIS A or JIS B, in the US I think they're called A4 and A5? In the UK it's something different I think. What I would recommend if count roughly how many characters fits on your target paper size, and just calculate the number of pages via the character counter at the bottom of the interface.
Good luck with your publication, let me know if there's something I can do to help :)