can I have mulitple apple ID's backed up to itunes?
This is a very common question and really there is no correct answer. iTunes belongs to the computer so you wouldn't run into an issue with less than five different accounts and even hundreds of devices would all sync with one PC and one iTunes library. 

When I worked at a certain fruit stand, I would ask people to imagine for a moment sharing a sock drawer with the person(s) in question. Some people like their socks sorted by size and color or prefer them folded this way or that. Some people live on the beach where it's warm year-round and don't even need socks. Some like to keep everyone's stuff organized, others need space to do their own thing. 

That being said, my default answer is to make each person their own user account on your computer and only connect one person's device to their account. 

That way you can choose to share photos using a photo sharing option or share app purchases using family sharing or share music using home sharing. Throwing everything into one large pile is generally something that people regret and having things separate usually doesn't cause any practical problem whatsoever.  At worst, when you run into a practical problem, you'll have many options to solve that problem without throwing everything back together. Having things apart makes the problem clearer and the resolution smaller work to accomplish.

PCs work with multiple user accounts just about as easily as a Mac does in my experience, so I don't see any reason to deviate from my advice to Mac users if you're using the Windows operating system and iTunes. Also, you could try mixing everything if you make good backups of all the things before trying that option.