A Dutch company has two numbers that are easy to mix up, and they answer different questions. One says the company exists. The other says it accounts for VAT. A supplier can have the first without the second, and the difference occasionally matters on an invoice.
The KVK number: the company exists
The KVK number is eight digits, assigned by the Kamer van Koophandel when the company is entered in the Handelsregister. Every registered business has one, from a one-person eenmanszaak to a listed naamloze vennootschap.
It is permanent. It does not change when the company moves, renames itself or switches legal form, and it is not reissued to another company after this one is struck off. That is what makes it the right thing to key your own records on — a name is not stable, and a company may trade under several.
A related number you will also meet: the vestigingsnummer, twelve digits, which identifies one branch rather than the legal entity. If you are checking who you contract with, you want the KVK number.
The VAT number: the company accounts for VAT
The btw-identificatienummer takes the form NL, nine digits, the letter B and two more digits. It is issued by the Belastingdienst, not by the KVK, and only to businesses that are registered for VAT.
Not every registered company has one, and that is not in itself suspicious: a company below the registration threshold, or one whose activity is VAT-exempt, may have a KVK number and no VAT number at all. What would be a problem is an invoice charging you VAT from a company that has none.
How they relate
They are issued by different bodies for different purposes, and you cannot derive one from the other by inspection. Treat them as two independent facts about the same company, both worth recording.
Checking them
The KVK number you check against the Handelsregister: it should resolve to the company you think you are dealing with, at the address on the paperwork, and be active rather than dissolved.
The VAT number you check against VIES, the European Commission's VAT Information Exchange System. VIES answers a narrow question — is this number valid and currently registered — and that is exactly the question you need answered before you deduct input VAT on the strength of it.
Both checks take seconds and neither needs an account. Doing them when you onboard a supplier, rather than when a payment goes wrong, is the whole trick.