noun, noun which may take the genitive case particle 'no'
per-person portion; for the number of people
Used when dividing or preparing something so that each person gets an equal share, or when the total amount corresponds to the number of people. Often appears with の before a noun.
I'll reserve seats for the number of people.
Please cut the cake into portions for each person.
分 is a suffix meaning 'portion' or 'share'. 人数分 combines 人数 (number of people) with 分 to mean 'portion corresponding to the number of people'.
Compound of 人数 (にんずう, 'number of people') + 分 (ぶん, 'portion, share').