For those who have the same question, here's the answer: if you have chosen 'EEA National Identity ID Card' as the document, you should enter the card number of your identity card as the document number.
On a Belgian identity card, you can find this number on the front side of your card, usually at the top. The card number consists of twelve digits and is often labeled as "Kaartnummer" or "Numéro de carte". It is usually mentioned next to a barcode and below the personal details, such as your name and date of birth.
On a Dutch identity card, you can also find the card number on the front side, but it is located at the bottom right. The card number consists of nine digits and is usually displayed under the text "Documentnummer" or "Documentnr."
If you have chosen your passport as the document, you should enter the passport number as the document number. The document number on your passport can vary in length and format, depending on the country of issue. Generally, the document number consists of a combination of letters and numbers. In Belgium and the Netherlands, the document number is usually located on the page with personal information, at the top right next to your photo. On mine, it literally states "Paspoortnummer / N° du passeport" above the required document number.
I hope this blog post has been helpful, and you can now travel without any worries. Have a safe flight!