I am about to scan my photo collection. To save time, I would like to scan multiple photos at once. How can I automate the splitting of photos in a single scan?
Photoshop and the cheaper Photoshop Elements both have options that can take one large image and automatically crop out sub-images. In Photoshop, the command you're looking for is File → Automate → Crop and Straighten Photos. In Elements, it's Image → Divide Scanned Photos. For more details, read this and this.
If you don't want to spend the money on Photoshop, you can download ImageMagick, and use this script to perform the separation. However it's worth noting that it's a command-line tool, so some familiarity with the command-line would help, although if you read the instructions on the script page carefully, that should be enough to get started.
Whatever tool you use, when you're scanning your photos, make sure you leave some space between them, and use something that will give a high contrast background, so the software will have a better chance of picking up where one photo begins and another ends.
The builtin "Image Capture" in macOS does this just fine. Just set "Auto detection" to "Detect Separate Items". See here for a more thorough explanation: https://appletoolbox.com/how-to-scan-photos-images-by-using-iphoto-on-a-mac/
Another 3rd party app for this: SnipTag. Like Photoshop, it can crop 5-6 photos scanned together. Additionally, it can batch crop scans: upload a batch of such multiple scans, and one click crops them all. It supports JPG, PNG, TIFF formats and any DPI setting. It's cheaper than Photoshop, and also includes a metadata editor (I actually downloaded it for batch-editing metadata, and discovered the auto-cropping feature later.)