Maximum Number of Photos

Don't confuse Photo Stream with iCloud Photo Library, they are two different things. Photo Stream is a short term share, and that is the last 1000 photos or 30 days. Take a look at this support document to see the differences between the two. About iCloud Photo Library and My Photo Stream - Apple Support

This is the only support document that I can find regarding restrictions in iCloud. Limits for iCloud Contacts, Calendars, Reminders, and Bookmarks - Apple Support and it does not indicate anything about photos. I'm guessing that it would be based on storage for both iCloud and the phone.

I'm glad that helped. iCloud Library is just a syncing service, and the photos actually reside in iCloud and are synced to all of the devices with the same iCloud ID. Deleting photos from one of the synced devices will delete the photo from all devices, and iCloud. That is where a great many users end up in trouble, and cannot figure out what happened to their photos. They tend to do this when running out of room on their phones, and think they have moved everything to the cloud and they delete photos on the phone for space and all of a sudden realize they have deleted their entire photo library. Good luck and enjoy your device(s).

I can't say, unless there is something in the description of iCloud photo. iCloud Photo Library - Apple Support