I upgraded to the new operating system and I see that iPhoto is no longer supported. I have over 20,000 images saved - what do I do to locate them?

Use the successor to iPhoto, the Photos.app. It comes preinstalled as a part of the system.

When you launch Photos the first time, it will create a new Photos Library from your iPhoto Library, and you will find the photos and albums in Photos.

If Photos should open with an empty library and it could not find your iPhoto Library, quit Photos again, then hold down the options key ⌥ while launching Photos. Releas ⌥ when you are seeing the Library Chooser dialog and select the iPhoto Library you want to migrate to Photos.

Here is the online user guide for Photos: Welcome to Photos - Apple Support

Hi

You need to migrate your iPhoto library to the Photos app.

If you don't already have a backup of the iPhoto library, make sure you take one first (the migration can (rarely) go wrong.

When you have a secure backup of the library, open the photos app, while holding down the option key. If the iPhoto library is shown in the resulting list, select it and click open.

If it is not, click "other library" and locate it from the browser. The migration will start automatically.

Bear in mind - with 20,000 images it will take a long time to update, and sometimes the progress indicator doesn't move for a long time. You'll need to be patient, AND leave your Mac on (connected to power) until it completes.

Use the Photos app. Hold down the option key while opening Photos and then select your iPhoto Library. Photos will open and convert it.