I just moved several thousand older photos that I had on Google Photos to Apple Photos in the same manner. It just took a few mouse clicks and was done. Apple even kept me updated on the progress. It took about three days. It appears that there is no limit provided that you have sufficient iCloud storage.Interesting. This isn't something I'd have expected Apple to do, or Google for that matter. But welcome nonetheless for those folks who move from platform to platform. Having ease of transfer and options available is always a good thing, I think.
https://support.apple.com/en-us/120924#:~:text=You can request to transfer,directly from Google to iCloud.