I think one or two short ones are fine so guests have a breather and can get a drink if they don't feel like dancing a slow song or don't have anyone to dance it with. However, this can clear the dance floor depending on the crowd and having a good DJ be able to read the crowd and fade out the music to a more upbeat song if needed is very helpful.
We didn't have any slow songs besides our first dance. This gave my MIL and FIL an excuse to not dance at all! Definitely know your crowd. In hindsight, I would request at least a couple for our older relatives in attendance.
I would put a slow song, 4-6 upbeat, slow, 4-6 upbeat. Lots of people (generally a little older) would prefer to slow dance so I think it's nice to sprinkle them in at least for the first half of the dance portion!