You don't need to know the project number if Apps script is managing it for you, which is the case for most Apps Script projects.
You only need to know it if you are associating your apps script with an alternative, existing cloud project through the project settings dialog in Apps Script because you want to use some special cloud platform APIS
Apps Script has been managing cloud projects for a long time now - so it's most probably just a temporary bug with creating new cloud projects - not a change.
As a workaround, you can re-use an old apps script (if you have one) which will already have a project assigned to it. If you don't have an old apps script you can use for now, then you'll have to monitor the problem to see it fixed
As you can see from this issue tracker link, many people are having the same problem today.