I have 12000 euro as cash when entering Europe. Any problems?

You need to file a declaration in Amsterdam. Relevant example from belastingdienst.nl

Example: flight from Tokyo to Paris via Amsterdam

You are flying from Tokyo (Japan) to Paris (France) via Amsterdam (the Netherlands). You arrive in the European Union in the Netherlands.

• If you leave the flight in Amsterdam (the Netherlands) (transfer), you will have to file the declaration in Amsterdam (the Netherlands). You can use a copy of your declaration to demonstrate in Paris (France) that you have filed a declaration in Amsterdam (the Netherlands) for the liquid assets.
• If you do not leave the flight in Amsterdam (the Netherlands) (no transfer), you will have to file the declaration in Paris (France).

The question that remain is how to do it at Schiphol airport. One option is to leave airside to get to customs and then enter through security again. You could look and ask for other options though.

You must declare that you carry more than €10.000 once you pass Customs. This is likely to happen in Poland.

Netherlands and Poland are in the Schengen Area, so you enter the EU when you first land in Amsterdam.

You have two options to reach Poland then: either by car/train/bus, leaving the airport, or by transiting into the domestic flights area. I understand the second.

Unlike Turkey, almost all Schengen airports do not have separate terminals. In Turkey, for example, you will have to go x-ray controls once again to board a domestic flight coming from outside. In Schengen, you first go Immigration, where you are only asked about your passport, and then you will have the option to either go to Baggage claim (through a no-way-back door) and exit, or remain in the Schengen departure area.

Once you land in Poland, you will finally go to the exit and there you will find Customs. You must go to the red lane yourself, or face the risk someone stopping you and confiscating the money.

If you need to collect baggage in AMS, e.g. because you are not allowed check-through, then you must go through Customs in AMS, but then you will have to make sure that your check-in bags are labeled again with a green label grant free access to the Blue Lane of Poland's Customs office.

Sources:

• Dutch Customs:Filing a declaration of liquid assets
  • download the declaration form, PDF, English

Please note!

• It does not matter how you enter or leave the EU (examples: airplane, car, motorcycle, ship, bicycle, on foot).
• It makes no difference how you carry the liquid assets, e.g. as hand baggage, hold baggage or in the car boot.
• You must also file a declaration if you are not the owner of the liquid assets.
• You must file a declaration each time that you enter or leave the EU.
• You file a declaration in the country where you enter or leave the EU.
• You do not need to file a declaration if you do not leave the flight or ship on which you enter the EU.

On arrival in the Netherlands from a non-EU country
File your declaration by reporting to Customs in the so-called 'red channel' (goods to declare). Inform Customs that you wish to declare liquid assets.