Missing Photos after airdropped from Mac to iPhone
There are two likely culprits:


The transfer didn’t complete and then the clean up didn’t happen.
The transfer did complete, but the library catalog to show photos is corrupt or was interrupted.


Both cases, the primary way you can clean up is to power down the iPhone - slide to turn it off and let the cleanup / shutdown process run. Then at start, the library often will run a sanity check on next launch and show you what the library has received.

If you were sending hundreds of photos, it’s quite likely the transfer started and once you saw the visual indication that the transfer started, you closed the sending window before the send was complete. When training people to AirDrop, most seem to think they are done when the transfer is just getting started.