ひとり
One Person
ふたり
Two People
さんにん
よにん
~にん
Three People
Four People
the default counter for any more people than 2
なんにん
How many people?
せんせいが ひとり いました
There was one teacher
せいとが さんじゅうにん いました
There were thirty students
がっこうに せいとが よんひゃくにん いました
There were 400 students at the school