I don't know what you want the numbering to accomplish, so I don't know if these methods will help.
For a visible number on the photo, you can annotate the photo using Photos builtin Markup tool, or another markup tool like Annotable. Open the photo and choose Edit. In the Edit screen/mode choose the (...) three dots icon near the top right. To use apple's tool choose Markup. If you have Annotable or other markup tools on the iPad, they should show up in the icon row.
You can also use third party markup tools from the share icon (square with up arrow). It is available when the picture is open, or if selected from from another view. Drawing apps may also be used, but are not optimized for this kind of work.
Apple's Markup tool does not have a text tool, so you have to draw the number with your finger or stylus/pencil.
Annotable has a lot of tools, making it easy to add text, tools, shapes, and more.
Annotations on the photos can be reversed/removed so long as it's the original and still in Photos. Copies, exports, etc. may make the changes permanent.
It's also possible to add captions to photos (as of the latest version of iPadOS. Open the picture in full view and swipe up. However, the caption only shows in this view. It won't display in album or other views. Captions do show up in search results though, making it a good way to tag photos for easy searches. And, of course notes on what the photo is about. Not sure if numbering would have any use.
If it's about the order of the album. In albums that you create, photos can be arranged by dragging them into any order you want. Automatically created albums like the main library and Selfies are arranged by the photo's creation date, and can not be re-ordered. Well, short of changing the photo's date. There are tools for that.