how can I see the total number of photos

Hi sAE327,

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.

The total number of photos might be a bit higher than shown, if your photos are RAW&JPEG pairs. The pairs are stacked and each pair is counted as one photo. The Burst photos are also only counted as one photo. The Photos in Shared albums are not counted in the total count at all.

The photos in the Recently Deleted album are also not included in the total. (tested in Photos 6 on macOS 11 Big Sur).