How to open a file with automator and then open another file when the first file is closed?
You can use the open command with the -W option to wait until you close the program. You can hardcode the list in an Automator Run Shell Script action where your command lines would look like the following:

open -W ~/Documents/firstSpreadSheet.numbers
open -W ~/Documents/secondSpreadSheet.numbers
open -W ~/Documents/thirdSpreadSheet.numbers


Note that you must completely exit out of Numbers (e.g., cmd-Q) for the next file to open; closing the window is not sufficient.