You can place as many photos as will fit on a flatbed scanner.
If the scanner has a TWAIN driver, you should be able to scan direct to Photoshop or Photoshop Elements using File|Import.
With most TWAIN drivers, you can preview and select individual photos and the scanner will scan each selection separately, and save them as separate images. With a scanner that only lets you make a full platen scan, it's easy to clip out the individual shots in Photoshop or Elements.
When placing multiple photos on the platen, it's difficult to keep them all accurately "squared up."
Hint, for Photoshop Elements (and probably for Photoshop), is to open the "Info" toolbar, select a "line tool" and draw a line along what should be a horizontal or vertical.
As long as you hold the mouse button down, the Info window will show an Angle (A) that you can read - but note that the angle disappears when you let go of the mouse button. Ctl-Z will delete the line you drew. Then Use Image|Rotate |Custom, type the correction angle in to rotate right or left as needed. You'll probably want to crop after you finish straightening.
There is an automatic Image|Rotate|Straighten and Crop, but it makes lots of mistakes. Use Ctl-Z to undo - frequently, if you try to rely on it.
If you scan a full platen and need to separate the pics, use Edit|Cut and then File|New from Clipboard. The new image will be "off the base layer" so you can't save it as .jpg until you do an Image|Flatten, since PSE can't save multi-layer .jpgs. For the last pic from the platten, just use Crop.
The Hint when cutting out individual pics is to set the background color "a little off" so that the area where you cut from shows in a distinctive color. (The setting doesn't affect the color of the pic you cut out, or of the remaining pics.) This makes it a lot easier to keep track of where you are, and is especially helpful when you pull the staples out of a pamphlet and scan two page (or multi-page) "sheets" and need to separate the individual pages to put them in order. (Select any drawing or paint tool and hit D to return to default foreground/background color.)
My newest multipurpose (HP OfficeJet Pro 8500 Premier) has an automatic document feed that lets you flop down up to 35 or so sheets and it will scan them all directly into a single .pdf document (with pretty good OCR conversion); but it "hard defaults" to 200 dpi which isn't really good enough for photos, especially if you might want any enlargement. You can override the resolution, but it whines about it and has to be reset for each "job." It's intended for record archiving, and my purpose in getting it was that things that go through the document feed will scan pages that are "legal size" - both sides automatically if you like, even though the "plate" only handles Letter/A4 size as a flat scanner. (The document feed will handle pages of mixed lengths in a single document, but they need to be close to the same width to feed well.)
There are "large platform" flat scanners but ones I've identified are relatively expensive (for what they do), take up a lot of desk space, and/or are sometimes limited to "record keeping" resolutions. Even the ones that claim high resolution often will only scan "itty bitty selections" at those higher settings due to internal memory limits.
For printing photos back at "same size" a scan resolution of 300 dpi is probably good enough, but you'll want finer scans if you might want enlargements, or if you may need to do much cropping. If you intend to use higher resolutions, you'll probably want a dedicated (external USB?) hard drive (or at least a separate folder on a large drive) to assemble the collection. File sizes can be very large.
PSE allows you to select the amount of compression when you save a .jpg. At high resolution/low compression (level 8) you're unlikely to see image degradation with a reasonable number of edits/re-saves; but of course you should always keep the original intact. You can go to "level 12" but a level 12 .jpg (zero compression) usually has a larger file size than a .psd since the .jpg "header" has to be added.
Also note that many scanners set to scan "black and white" will save only as indexed color or bitmapped images that many editing programs can't save as .jpg files (.tiff is a common default). I generally scan everything as "color" and if necessary for a later use convert a copy to grayscale after scanning; but your equipment will determine whether that's your best practice.