분 has two uses as a counting word.
1. respectful version of 명 or 사람, like "gentlemen" or "lady" but without referring to the gender.
e.g. 일행이 몇 분이세요 = How many people are there in your party? (more respectful than 몇 명/사람이세요?)
분 also makes pronouns like 이분 (this gentleman/lady), 저분 (that gentleman/lady), 그분들 (Those ladies and gentlemen), etc.
2. suffix referring to a specific apportioned quantity (usually follows a word describing the amount)
- 1인분 = quantity for one person (as of food).
- 30일분 = a quantity to last 30 days.
- 원정대는 50일분의 식량을 준비했다 = The expedition amassed food for fifty days.
* It also means "minute", as in 30분 = thirty minutes, or 12시 20분 = twelve twenty (12:20).