The formula field is displayed on the opportunity detail and edit pages, and you can see how this appears as week number 3 for a Close Date of Jan 16, 2020, as shown in the following screenshot:
The formula field is displayed on the opportunity detail and edit pages, and you can see how this appears as week number 7 for a Close Date of Feb 14, 2019, as shown in the following screenshot:
The week number calculation is set to return week numbers for close dates greater than January 1, 2017. Is envisaged, at the time of writing, that dates earlier than 2017 would not be valid close dates. If your business rules require the presentation of week number values prior to 2017 you will need to adapt the Salesforce CRM formula for this recipe accordingly.