Finding your catalog ID is very simple.
First, within Facebook you can open the Meta Commerce Manager here.
Once you are connected on your account on that page, open your business assets (they should be visible right under teh commerce manager).
This will open a screen with your Commerce Assets, namely your Shops and your Catalogs.
In the list for Catalogs, under the name of the desired catalog you can find the Catalog ID which is composed of 15 numbers.
It should look something like this: