do you need a separate apple ID on different iPhone in your family

As noted by muguy, each person should have their own ID. That's the only way to maintain any kind of individuality. If you use a single Apple ID on multiple devices (which is possible) then iCloud will try its best to make sure everything on each phone is kept the same as it is on other phones. If you add a file, other family members will see it. If a family member deletes it, your copy gets deleted too.

How to create a new Apple ID - Apple Support

Use Family Sharing to be able to share the things you do want to share. This article tells you how to Set up Family Sharing

Each person should have their own Apple ID. To set up family sharing: https://support.apple.com/en-us/HT201088