Sooner or later, most writers seem compelled to opine on their personal "formula" for writing. Most of them claim to write first thing in the morning. All of them demand that a minimum number of words be written each day, usually between 2,000 and 5,000. That's ridiculous. Even if you have no day job requirements, family responsibilities, or outside activities or interests, it still would not be realistic to expect to achieve a minimum daily word count.
I write techno-political thrillers. The stories include the use of cutting edge technology and weaponry, and take place in various spots around the world. I have to research the technological devices and weapons. I have to read materials detailing the language, history and culture of areas I haven't personally visited. I have to study maps and aerials of these places, determine distances between points and often convert them into kilometers or other system of measurement. I have to research the surnames and given names of people in foreign lands to be certain I don't give a Polish name to a Lithuanian character in a novel. Sometimes, I have to interview people to get the information I need for a storyline.
Unless you're writing your memoirs, the effort to write accurate, believable fiction requires a great deal of research. Some writers, like Jim Rollins, claim to do most of their research before writing the novel. In that instance, perhaps you can set a goal of so many words per day. My experience, however, is that new elements surface throughout the writing process that require additional research. And it can take up an entire day or more, thus eliminating any word productivity for that day.
My advice: don't set yourself up for disappointment by trying to meet a daily word goal. But do make it a point to work on the book in some fashion everyday.