I planned to go to Europe for tourism for 12 days, 8 days I will be in Amsterdam and Rotterdam and 1 day in Brussels and maybe Spain the rest of the days, so I will apply for Schengen visa Netherlands through VFS, but I found an interesting Programming conference for 3 days that happing during my stay there, but it's in Copenhagen and I bought the tickets and they sent to me an invitation letter. my question is, should I attach the invitation letter for the conference that happening in Copenhagen and mention that I'm staying in Copenhagen for that?
Normal procedure for a Schengen visa is to apply at the representation of country which is to be your main destination. By days of length it is the Netherlands for you. BUT on paper the reason for your trip would look like the conference in Denmark.
Source (emphasis is original to text):
You have to apply for the Schengen visa at the Embassy/Consulate of your MAIN destination. The MAIN destination is the country where you are staying the longest time or where the main purpose of the trip is carried out.
Now, the Dutch representation in the US says this about visas (emphasis mine):
A short-stay Schengen visa with ‘Tourism’ as the purpose of travel is only for visiting the Schengen area as a tourist. This includes as part of a holiday, a cruise, city trip or other recreational purpose.
If you wish to combine such a visit with a business trip to the Netherlands, select ‘Business’ as your purpose of travel.
A short-stay Schengen visa lets you visit the Netherlands for a maximum of 90 days within a 180-day period. It is also valid in the other Schengen countries.
Your conference would, in my opinion, be considered business. It would be rather hard to convince the Dutch authority that your trip is tourism mostly in the Netherlands with a side of business in Denmark.
I would recommend contacting the Danish representation in your country. It is more likely for you to convince them that this is a business trip to Denmark with a side of tourism throughout the Schengen area.