Q: how many iPhoto libraries can I sync with iCloud?

Yes, see this -- iCloud: iCloud storage and backup overview

One shouldn't consider using the iCloud Photo Library as backup. It is primarily a syncing service providing you with the convenience of having your photos available across all your Apple ID-controlled devices.

You could back up the library databases to iCloud Drive, but once again, this is not a core strength of iCloud Drive. I say that because at present, the local files will still be on your local drive as well as in the cloud.

Not moved to iCloud. Synced with iCloud.

LACAllen I sure do wish I could pick your brain as I've got some interesting questions for you!  I do like your writing and cut of you gib!

Thanks. I often get tagged with being the opposite.