I voted 4, but its a slightly stacked question because it depends on the blade. But for me with my daily setup (Slim adjustable with a Persona Red) 4 is the ticket. I could do more, but I like a fresh blade after that.
Usually get 4 shaves from a blade, sometimes I'll push it to a 5th if the 4th was still smooth...that's with Astra Platinums or Merkur if I have those laying around.
Current Personna 74 didn't even break in until 5-6 shaves and tonight was shave #24 and yet another BBS..
Yes, they are that good and I will be bummed when I use my 2 up.
I can't vote as my pick is not available: "10 or more". I consistently get 2 weeks of shaves out of my blades. Current favorites are Ladas, Voshkods and Rapira with a Super Iridium now and then and a Polsilver for good measure. But pssst ... I use a secret sharpening device called magnet, don't tell anybody .
Dorco = 6
Derby = 8 (yeah, I was surprised by that, too)
Feather = 6
I use a blade until it starts to pull when shaving ATG. I have wire whiskers, so I'm surprised I got 8 shaves out of Derby. I think it's partly due to finding the right razor with them. When I tried Derby blades with one razor, they hurt the first time I used them. Tried a different razor, and they work great!
I can use my Feathers in only one razor, my least aggressive Tech razor (pre-40's).