What if most of my photos only show up on my Mac?

To see your photos on all devices you have to turn on iCloud Photos on all devices and use the same AppleID on all devices.

On your Mac you can have several Photos Libraries, But only one of your Photos Libraries can be syncing with iCloud Photos, and this library has been enabled as your System Photos Library. See: System Photo Library overview in Photos on Mac – Apple Support (UK)

To make the library you want to sync with iCloud your System Photos Library, open this library in Photos then open the Photos > Preferences > General. Click the button "Use as System Photos Library", if this button is still active and not dimmed. Now you can enable iCloud Photos for this library as described by dawngreg08. Enable iCloud Photos in Photos > Settings (Or Preferences, depending on your system version).

To see, if the upload to iCloud is successful, watch the Status bar in Photos. Here you will see, if there are problematic ites, that cannot be synced with iCloud, see: Check the status of your library in Photos for macOS - Apple Support

Hello smithjw16,

Welcome to Apple Support Communities!

First let's address the issue about Photo Stream shutting down and if you'll lose your photos. The article you came from proves the information your need in order to save your photos and that is to ensure you save them to your device and then set up iCloud.

"Save photos currently in My Photo Stream

If your photos currently in My Photo Stream aren’t already in your library, you can save them to your device.

On your iPhone, iPad, or iPod touch

1. Open Photos and tap Albums.
2. Tap My Photo Stream > Select. 
3. Tap the photos that you want to save, then tap the Share button  > Save Image.

On your Mac

1. Open the Photos app, then open the My Photo Stream album. 
2. Select any photos you want to save that aren't currently in your photo library.
3. Drag them from the My Photo Stream album to your Library. 

Set up iCloud Photos

You can turn on iCloud Photos on any iPhone with iOS 8.3 or later, iPad with iPadOS 8.3 or later, or Mac with OS X Yosemite or later. After that, you can view your photos and videos in the Photos app on your iPhone, iPad, Mac, Apple TV, iCloud.com, and even sync them to a Windows PC using iCloud for Windows. 

Learn how to set up iCloud Photos on all of your devices"

Information about the My Photo Stream shutdown

The issue with your photos not syncing across your devices, you'll want to follow the steps in this article: If your iCloud Photos aren't syncing to your device

Let us know if you have any questions.

Thank you for using Apple Support Communities.

Take care!