How many km? I never really planned that way – in fact, I never really planned any of my tours. If I need to plan a stretch, I might guesstimate approximately 120-150 km per day depending on hills, wind, etc, and I know that if I don't do it one day, I will generally be back to that average over a week or so.
But one I look back at my trips, I find that the most reliable piece of data is the time spent "riding". When by myself, it's 12 to 14 hours, including lunch, rest stops (about 2 hours total), vistas and other interesting diversions. Which means that on flat and uninteresting terrain, it's 10 hours x 20 km/h or 200 km, but that on rolling terrain and/or places with museums and the like, it may be as little as 50 km because I spend the rest of the time doing NOT cycling.
Now that I tour with one child (two this Summer), it's more like 8-10 hours per day, stops included.