How can I recover deleted photos on my iPhone?

You didn't give much information. Did you look in Recently Deleted? Do you use iCloud Photos? Do you use iCloud Backup? How long have they been gone?

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.

Unfortunately, there is no "extra" safety feature in case we didn't heed the warning for the first safety.

You can see this:

        If you're missing photos or videos in the Photos app

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. 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. You would have to get to the backup quickly before it's replaced by a newer one. However, if you did have iCloud Photos enabled, then the iCloud Backup wouldn't make an extra copy.

Unfortunately, if you don't keep your own backups to an external drive, then there really isn't any place else to look. To back up your pictures you can just plug in an external drive, select the pictures, and use Share>Save to Files. Alternatively, you can backup to a computer:

Transfer photos and videos from your iPhone or iPad to your Mac or PC

If you haven't done one of those backups, then your remaining pictures continue to be in peril.