How do I find out how many photos, videos are stored in iCloud?
Open the Photos app. In the bottom-right corner, tap Albums, then the top-left album named All Photos.

At the bottom of that album, you will see the exact number of photos and videos in your photo library, including any Hidden photos.

Optimize Storage doesn't mean that photos are never downloaded from iCloud. iOS will download higher-resolution versions of your photos and videos as they are viewed/zoomed/edited/shared. These higher-resolution photos will not be purged until space is needed.

It is normal for the amount of storage taken up by the Photos app to grow over time. On my 128 GB iPhone, I have 60 GB of storage taken up by photos and videos (out of a 140 GB library), even with Optimize Storage selected. But since I also have 30 GB free, there is no need for iOS to delete anything yet.