(Can be reduced or increased without changing script length as alternatives are offered for the narrator or other actors to say if you omit any parts.)
If used as written it will include narrator (s) and 18 speakers, plus non-speaking extras and singers, as follows:
Narrator, 2 roman soldiers, Mary, Joseph, Donkey, shepherd 1, shepherd 2, shepherd 3, Angel Gabriel, innkeepers 1, 2, and 3, , innkeeper’s wife 3, innkeeper 4, star, kings 1, king 2, king 3, plus angels.
(All the parts in italics are non-crucial. Other parts can be doubled up, and optional extra walk on parts can be included for cow, sheep, innkeepers wives 1, 2 and 4, Caesar, crowds of people at the inn, carol singers.)
My own expectation would be that a minimum cast of 10 would be preferable, although the smallest cast to date to use the script (to my knowledge) is 5! The aim of all of my scripts is to be adaptable and flexible such that anyone is able to put on a nativity play, so cast size need not be an issue: just be inventive and, if all else fails, email me for some advice.