Are you asking about the limit 2TB for iCloud Storage?
I would not let a Photos Library grow that large. The larger your library, the longer the background processes analyse the photos will take. Photos is not just managing your photos Library, it is also analysing the photos and the relations between the photos, scanning for people and pets, recognising objects and scenes, creating albums and memories, scanning for duplicates. even for moderately sized libraries this preprocessing can take weeks. my own iCloud library with just 65000 photos and videos is already trying my patience with the amount of background processing after each system upgrade, also when the library needs repairing or a new upload to iCloud Photos.
I'm continually weeding the items in my iCloud Photos Library out and are only keeping the items there, That I need on all my Apple devices, the iPhone, iPad, my Macs. The available storage on my smallest device (the iPhone with just 512GB storage) is anyway the bottleneck for the library size, as I do not want to have to fall back on "Optimize Storage", so I an seal photos on my iPhone, even if I do not have an internet connection.