Lost photos

Did you transition your iPhoto library to Photos?

Launch Photos with the option key held down. From the resulting dialog, choose your iPhoto Library. Photos will create a new Library with your iPhoto media.

iPhoto is a 32 bit app and will not run on Catalina. For about a year we would get this type of warning when we would launch a 32 bit app on Mojave or High Sierra:

When iPhoto Library Manage runs it has to have iPhoto open and close several times during the process. iPhoto doesn't work on Catalina.

If you have a Time Machine backup of your boot drive before the upgrade occurred or a cloned drive you can revert to your previous system by following this Apple document: Revert your Mac to a previous macOS version - Apple Support 

What you might be able to do is open the iPhoto library package and copy the Originals and Masters folders to the Desktop. Then import their contents into your new Photos library. You'll lose any edits or metadata added to the photos but should get the originals back.

Are you saying that you transferred your library from iPhoto to Photos and a lot of pictures were missing? It sounds like your iPhoto library was corrupted.

You might need to get the paid iPhoto Library Manager from FatCat to rebuild your library.

Unfortunately even though iPLM will run under Catalina it requires iPhoto to run also in its Rebuild process. It won't work even with the hacked version of iPhoto that does run under Catalina.