Actually, only 一人 and 二人 are different. All the people counting numbers use 人(にん) after one and two. Ex)
ひとり
ふたり
さんにん
よにん
ごにん
Etc
One person and two people are the only ones (that I know of) that use “り” to express the person.
Additionally, the reason for 七人 being read as しちにん is mainly for aesthetic reasons, if that makes any sense. Later on, you’ll probably learn lots of counters, and most of the ones that I have learned change はち to はっー With a small tsu in place of the ち for the ease of pronunciated and understanding. There really isn’t any reason that I’ve been told besides this in class. The same thing happens in French, words get cut, added to, and changed around in pronunciation solely to please the tongue and ear. Nothing grammatical, syntactical, or otherwise. Just for the ease of it.
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This reply was modified 10 years, 3 months ago by Chibifreak.