You can have up to 10 devices linked to the same Apple ID. This is OK if all these devices belong to the same individual. However, if different users use different devices, they all should have their own Apple ID. This is the only way to avoid confusion between what is synced to each device.
You can have different Apple IDs syncing to the same iTunes on your computer and it will keep track of the different preferences for each ID and each device.
The the best option, however, is to set up separate user accounts on your Mac, as well as separate Apple IDs. Mac OS X supports Fast User Switching. You set up an account for each user in System Preferences > Users & Groups. This way each user has its own "world" to live in.
Mac applications will be available to all users, if you want the other users to have access to the main iTunes, or Photos, library you can set up Family Sharing.
I'm throwing a lot of new things and terms at you all at once, but don't feel overwhelmed. Just perform a search on Apple support pages for each of these functions or terms. The help is out there.
