After finishing all the classes, you will automatically receive a certificate of completion that you can download.

Important to state is that it is a certificate just for you. It's not an official document that you can use for applying for a residence permit or a job. For those purposes, you need to take the NT2 exams. For more info: https://www.staatsexamensnt2.nl/item/state-exams-dutch-as-a-second.