On the ESTA form for entry to the USA they ask for a National Identification Number, which should not be the same as a Personal Identification Number.
What kind of number is it and where do I find it?
That'd be the BSN.
The Dutch Citizen Service Number (BSN) is your official national identification in the Netherlands. It replaced the old social security number (sofinummer) in 2007. The Dutch BSN was introduced in July 2007 to enhance the efficiency of government administration and improve public service delivery to citizens.
The BSN in the Netherlands is closely linked with the national register (BRP). Registering with the BRP is key to getting your Citizen Service Number in the Netherlands. Once you have registered with the BRP, you will be issued with a Dutch BSN which will be needed for any interactions with the Dutch government or public services, from receiving social security or healthcare to enrolling for study or paying tax. This means that the BSN functions as a social security number, a national identification number and a tax number in the Netherlands.
Whereas the sofinummer was the responsibility of the Dutch tax authority (Belastingdienst), the Dutch BSN is administered by the Ministry of the Interior (BZK) and issued by the local municipalities. It is a unique 8 or 9 digit number which can be found on Dutch passports, national ID cards and driving licences.