How many times did Germany try to negotiate a peace in WW2?

I don't believe that the German government ever attempted to negotiate an end to the war. Individuals did, which was actually more defection than negotiation. It was more the other way around. Germany betrayed agreements with other countries.

Peace with Netherlands after invading it.

That's not correct. The Dutch government fled the country, so there was no government to negotiate with. The commander in chief, gen. Winkelman was given authority to negotiate a surrender, not to negotiate a peace agreement. The German state took over governing the country, as it was required to do under international law.

Besides, peace negotiations assumes there is something to negotiate about, apart from 'please sign on the dotted line'. That was what the Dutch army (in lieu of the government), as well as the French and Danish governments had to do.

Well, you have omitted several countries that Germany conquered, including Belgium, Greece, Luxembourg, Norway and Yugoslavia. For your definition of peace negotiations, it's worth checking those.

Hess' intention to negotiate never stood a chance. He seems to have been operating under a fundamental misunderstanding of the British form of government. He imagined that the Duke of Hamilton, whom he'd apparently met briefly at the Berlin Olympics, could get him an audience with the King even if the government was opposed to it. Once he had that audience, he expected that the King would be happy to fire the whole government and install a new one that would side with Germany. This might make sense if you assumed that the British government worked exactly like that of Imperial Germany, including Kaiser Wilhelm II's personality flaws, and that most of the British people wanted to join Germany's side but the government was in thrall to Germany's Jewish-Bolshevik enemies. That conspiracy theory was at the root of Nazism, and tended to wreck committed Nazis' attempts to work with other political systems.

Richard J Evans' The Hitler Conspiracies: The Third Reich and the Paranoid Imagination, Penguin, 2020, which is about debunking said conspiracies has an account of Hess' offer. Germany would get back the colonies taken from it at the end of WWI, but the British could keep their empire if they gave Germany a free hand in Europe and made peace with Italy. The British were not at all interested in this: if Germany were given time to consolidate its conquests, it would grow stronger, make new demands and a new war would break out.

Also, Hess' credibility was destroyed when his mission was disowned by Hitler.

Himmler wanted to negotiate with the Americans and British, and certainly asked people in Sweden to arrange for negotiations. That seems to have been as far as it got, because news of it leaked out, leading to him being fired as a traitor by Hitler. Himmler's negotiating position is known, which was that he and the SS were vital to maintaining order in Germany, and that obviously the Western Allies would be happy to join with Germany in making war on the Soviet Union. Given the Allies' actual attitude to the work of the SS, this was never going anywhere. By the end stages of WWII, it was clear that the West and the USSR were going to have difficult relations afterwards, but Nazi Germany was worse, and they were going to deal with that first.

On the "peace negotiations during Dunkirk", the author in question, Stephen Davis seems to work mostly in the field of sensationalised history. Express.co.uk, where the story appeared, is a highly unreliable source. It's above the level of an American supermarket tabloid, but not by much. It concentrates on trying to make the memories of its elderly readership seem important, and in sensationalising anything that might help with the health problems of the elderly. That is its agenda. You can see how this story fits right into the first part.