As far as I know the collections shared from LR Classic to the Web/mobile system does not count against your allotted Adobe storage space. This might change in the future. But all images uploaded from a phone or pad, through the mobile LR version, does count against that storage space allotment. And they are Full Size. Not the Smart Previews that get uploaded from LR Classic collections. Also AFAIK there is no maximum number of images in synced collections or on the web interface.
I have read that some people place all their images in synced collections so all of them are available on the mobile and web LR versions. Even though they are Smart preview that would take up a lot of space on the Adobe cloud servers And that might be why Adobe is looking at limiting the number/space used through LR Classic.