I know Japanese does not count the same way as English, they do not say something like "I have four cats at home." instead it's 家で４匹の猫を飼っている (匹 is a counting suffix used when referring to smaller or medium sized animals like domesticated pets for instance.) This is known as 助数詞 (じょすうし) in Japanese.
By the way there are 350 counting words in Japanese, but nobody uses all of them, the only ones you will commonly spot are: 人 - Refers to the no. of persons within a setting or those present. For instance, you do not say 8たこ焼き, you instead say たこ焼き8個 - with the counting suffix included to make it clear to the speaker.
In terms of 一人 (it's pronounced as ひとり) while 二人 is pronounced as ふたり however from 三人 onward: 人 is pronounced with にん at the end, eg. さんにん. Some more (I won't list them all):
- つ - A counter for [things] in general, as it is also commonly used in Japanese.
- 枚 - Refers to the no. of sheets of (paper), pieces of flat objects (like cloth, credit cards, etc.)
- 階 - Refers to no. of floors within a building. (E.g. this apartment complex consists of 30 floors.)
- 冊 - Primarily a counting suffix used for documents or books (E.g. Nikolas read 3 books.)
- 件 - Refers to the no. of [cases / incidents] but this counter has versatility in its usage.
- 個 - Counter for [no. of pieces] or some things, you see this word in relation to let's say: food.
This is why this is hard for speakers of English to grasp, when they are learning Japanese since the manner in which they count numerical units always need an associated suffix tied to it. (Even native Japanese speakers only use a portion of that, you don't expect them to know literally every single one.)