Photos missing from album

What kind of album are you asking about? And what has happened, before the photos were lost?

Did you perhaps just add a photo to an album, and then delete the photo from a different album? Then the photo will have been deleted from all albums. The photos in the albums are not copies. There is just one copy of each phot in your library and all albums are showing the same photo. If you delete a photo from one album, it will be removed from the library completely, from all albums you added it to.

And if you are using iCloud Photos to sync your devices, all synced devices are using the same photos in iCloud. If you delete a photo on one device, it will be deleted from iCloud Photos and all other synced devices.

Have you already checked, if your missing photos are still in the Recently Deleted album? If the photos are still in Recently Deleted, recover them from there asap as described here: If you're missing photos or videos in the Photos app - Apple Support, they will only be saved there fore 30 days.

Below your question the system version iOS 12 is given. Is that the correct version?

If you are also having devices with iOS 16 or macOS 13 Ventura and enabled the shared iCloud Photos Library on one of your devices with the current system version, any video or photo you shared to your shared iCloud Photos Library will no longer be visible on the devices with an older system version. The Shared iCloud Photos Library is only accessible on devices with the the most recent system versions.