Numbers app on iPhone not syncing with iCloud
Solution 1:
In your iCloud Settings on your iPhone, System Preferences > Your Name > iCloud, there should be an option for Numbers (and Pages, and Keynote as well, if they're installed), which should be set to 'on'. The option is likely under iCloud Drive.
If you disable any of these for any reason, say something like Reminders, iCloud won't sync that data between your devices.