@digitalclips thank you so much for the information.
I think those numbers make sense. After all, it is only the initial upload of existing photos that will take time and be huge. After that, 1000 photos per day in Family Sharing, 100 shared albums, etc., sound pretty decent.
No doubt the numbers will go up once all the kinks are worked out.
I am already paying for 500GB iCloud storage in anticipation of storing photos on iCloud.