0 the number 1 -- 1, 1の
2 at a particular time in the past -- (過去の)ある(時)
3 a few -- ひとつ、ふたつ
I’d like to make one or two suggestions.
4 used to refer to a time in the future that is not yet decided -- (将来の)ある(時)
We should go out one night.
5 used when saying there is no other person or thing -- 唯一の
6 a single thing -- ひとつの
7 one person or thing in a group that has already been talked about -- (前出の物の)1つ, (前出の人の)1人
8 separately, one after the other -- 1つずつ, 1人ずつ
9 first one, then another, then another, and so on -- 次から次へと