It can be anything: word count, page count, minutes of writing (or hours of writing), number of ideas, cups of coffee consumed, an amount of times of hitting your head on the wall...you get the drift.
Once you set your goal, chip away at it EVERY DAY.
The reason for this is, if you do something very small every day, at the end of the year it'll be a huge thing (over 365 days!). And this is the only way you'll know for sure you're moving forward. Because you'll know you're working toward something bigger—something that's too scary and too hard to accomplish all at once.
Not if you chip away at it. Again. And again. And again. Little by little. Word by word. Page by page.
You will get there.