Back when Google introduced Google Photos at this year's IO developer conference, it was listed as an unlimited photo storage service which users could place "unlimited" amounts of photos into to back them up or use as main storage. Unlimited means unlimited, but according to Android Police a number of users have been reporting reaching an unlisted data cap for their photo storage when uploading their pictures into the new mobile app and the desktop uploader program. At the time of the announcement it seemed the only limitations that Google Photos had was in regards to the original image format. Users would be allowed free and unlimited storage of photos, but there would be compressed to a series of formats and the original format would no longer be available with the free version.
However, some of the reports from users claim that while uploading all of their pictures to the new service from Google, uploads would unexpectedly stop in the middle of the process, which would seem that users had reached a limit to how much they could store. This invisible cap as it's being referred to doesn't seem to be caused by the amount of photos you upload, but rather by the amount of data you use. What's more, when and if people reach this limit, photo uploads seem to stop across all user devices which are connected to a Google account and support photo uploads with Google Photos, i.e. desktop, smartphone, tablet etc.
Paying the monthly fee for a Google Drive storage amount seems to lift or raise the limit and allow the user to resume photo uploads, but depending on how much data is being used it seems possible to again reach another limit depending on how much data you're paying for. One example describes a user beginning with the 100GB storage option in Google Drive, after which the photo uploads began again, only to reach the limit once more and then proceeding to upgrade to the 1TB option for Drive for uploads to resume. While Google Photos is free and it should be expected that there may have been some limits in place somewhere, it doesn't appear that any description of this particular situation is listed in the terms of service which leaves users uninformed only to be surprised if they're met with such limits.