I'm not sure exactly. It depends on how you count different editions and volumes. You might need to check with reliable manga databases or fan communities for a precise number.
The ending of Assassination Classroom in the manga is quite touching and conclusive. The students manage to fulfill their goal and there's a sense of closure for all the characters.
In general, Assassination Classroom Season 2 is quite faithful to the manga. Some scenes might be presented differently for visual or narrative effect, but the core story and character developments are consistent.