Small question: In my Japanese course I've come across 一人で (or something like that) meaning "by myself", but I can't find out how to pronounce it, since Duolingo isn't pronouncing it like "ichi gin de" like I would expect it to. How is it pronounced?
人 has several readings: ひと (kun-yomi) and り、にん、じん (on-yomi).
The sequence for counting people: 一人（ひとり）、二人（ふたり）、三人（さんにん）, then continues the pattern 'number + にん）. 数人（すうにん）is 'some people'. 人数（にんず/にんずう）is 'number of people'. 何人（なんにん）is either 'a number of people' or 'how many people' if used in a question.
As 'Gi' and 'ji' have totally different pronunciations in Japanese, try to avoid writing じん as 'gin'.