You don't say if you're using iCloud Photos, or iCloud backup, or what. It's important to keep backups of our most important pictures and work on disk drives that we control. A great safety feature on the iPhone is that deleted pictures remain in the "Recently Deleted" view for 30 days before they are removed permanently. This safety feature can be defeated, though, by choosing to prematurely delete pictures from the "Recently Deleted" view. If you use iCloud Photos, then that "synchronizes" its Photos Library with the phone, so what you see on the phone is what is at iCloud. iCloud Photos doesn't keep pictures in "Recently Deleted" any longer than the phone does. So it's really important to look carefully at the Recently Deleted pictures before deleting them permanently.
Some pictures transferred from a computer can be viewed in Photos but they can't be modified by Photos, so there may be some operations that produce a warning about those being at risk. So if you’re missing pictures that were transferred from a computer, then you should look on the computer for the originals.
You can see this:
If you're missing photos or videos in the Photos app - Apple Support
Basically the above says to look in Recently Deleted and in Hidden. Be sure to check your Library thoroughly-- the pictures may not be where you expect, as dates may have changed. You can use the search field with the date or location. And remember, you can sort the Library by date captured or by date added. Also, your Library view now has filters, and you may not have your filter set to "All Items.”
If you don't use iCloud Photos, but you have Settings>Apple ID>iCloud Backup turned on, then you may be able to recover data from the last 30 days or so. You should look here:
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
“Restore” is what you do when you buy a new phone, so this safety, if you have it set, requires you to erase everything on your phone and replace it with the backup.
Unfortunately, if you don't keep your own backups to an external drive, then there really isn't any place else to look.
