In an anime I've been watching, the protagonist goes to a village where one of his friends came from, where all the people in the village have the same hair color as his friend. (with "〇" being the name of his friend) He says "〇が一人、〇が二人、〇が三人...". English subs say "One〇, two〇s, three〇s...". He says this out of surprise of the amount of people that look like his friend due to their hair. I'm a bit confused as to the use of the particle "が" in this context. Is "が" taking the place of "の" or being used as some other sort of possessive particle? Is it a certain way of counting people? Or something else entirely?
This が is a plain subject marker, and it's not interchangeable with の. This 〇が1人 is a contracted version of 〇が1人いる. いる has been omitted because it's redundant when counting things.
In English, you count sheep like "One sheep, two sheep" (but not "There is one sheep, there are two sheep", which is too cumbersome). In Japanese, we say "羊が1匹、羊が2匹".
Grammatically, this 1人/1匹 is an adverb. Japanese people count things using adverbs by default. See: How to list numbers of things