How long has it been? Have you re-booted recently? (That can help.) It takes quite a long time, since Photos scans the pictures as they come in, and there's lots of work to be done organizing the scans. And this work can't be done when you're using Photos or when there's a danger of depleting the battery. So, if you leave the MacBook plugged in, turned on, with good internet and with Photos in the background overnight, you should see some results in the morning. You may need to go to System Settings and find Lock Screen to set it to not sleep. If the Mac says "Synching with iCloud" at the bottom of the Library view, then it looks like it's working.
If you don't get good progress overnight, they try a re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this:
Start up your Mac in safe mode - Apple Support
Let us know how it goes…