Numbers is not reopening my spreadsheets

If you want to be able to close Numbers when several spreadsheets are open and then later start Numbers and have all those spreadsheets open back up, there is/was a setting in System Preferences/General to "close windows when quitting an app". Deselect it.

It's not easy to find. I had to do a search in Settings to find it. The setting is not in System Settings > General, it's in System Settings > Desktop & Dock.

If you are using a version of macOS that has System Preferences, it is in General.

I don't have a recent enough version of Mac OS to have the settings Peggy showed. In previous versions it was in the General" section. It makes no sense to me why it is in Desktop and Dock now.