If you enter or leave the Netherlands, separate rules apply to money and gold.
How much gold can I take with me on a trip? That's up to you. But are you taking € 10,000 or more with you? Then you usually have to file a declaration with Customs. This also applies to the form of a "store of value" such as gold.
You do not have to file a tax return for amounts up to and including € 9,999.99 per person.
If you enter or leave the Netherlands, you do not have to declare it to Customs in the Netherlands on your own. However, Customs may require you - for example in the event of an inspection - to submit a 'notification declaration' of money, valuable goods and documents relating to assets with a value of € 10,000 or more. But you will hear that from Customs. You do not have to pay tax on the money at Customs.
Different rules may apply in other EU countries and you may have to declare to Customs when you enter or leave that country. Check with Customs in the country of your departure or destination.
Then you must declare cash to Customs.
If you are departing from an airport, go to Customs. Make sure you make a declaration before checking in your checked baggage and going through security. Are you arriving in the Netherlands from outside the EU? Then choose the 'red exit' when you are at Customs. This is the exit for when you have goods to declare to Customs. You can then file a police report there. Please note that you must also file a tax return if you enter or leave the EU by boat or car, for example.