How do I get some of my photos back from backup

Have you checked the Photos app and if you were syncing with iCloud, at iCloud.com/Photos using a computer browser for a Recently Deleted album? If you weren't syncing photos with iCloud, do you have a backup you can restore from? Restoring from a backup erases all data currently on your device. Any data newer than when the backup was made will be erased. A restore is an all or nothing restore. You can't access a backup and only restore certain files.

Backup - Restore your device from an iCloud or iTunes backup.    

I hate to say it, but it is not unusual to lose some photos if using an iCloud backup. The best thing to do is to sign into iCloud.com using Safari and a computer and see if any of the missing ones are there--in Photos or in Recently Deleted. If any are there, you can download them to the computer using Download iCloud photos and videos https://support.apple.com/en-us/HT209454