How can I share my storage with only one person in my family plan

You can share an iCloud+ storage plan with anyone in a Family Sharing group. Up to six people can be in a group; the organiser and five others.

If you are asking about limiting a shared storage plan to one other person, be advised that this cannot be done. Each family group member can use their space as they wish; it can't be given to some members and denied to others.

See

What is Family Sharing?

Set up Family Sharing