IMO, there's no type limit.
There are types that Ash always gets, grass, fire, water, and flying. Those 4 along with Pikachu take up 5 slots in Ash's party. His 6th pokemon is usually a wildcard that seems to be pretty random. There's so many pokemon out there and only 1 slot open for a dark type to make it onto Ash's team; odds are it won't happen. In Kanto, the way his team was handled he got plenty of different types, bug, fighting, etc. there weren't any dark types back then though. In Johto and Sinnoh he got ground types and in Hoeen he got an ice type, it's luck of the draw IMO.
I know someone is going to bring up Corphish not evolving. Why didn't Totodile evolve? Why does it seem like Buizel will more than likely not evolve? They do not evolve into dark types but they didn't evolve.
The reason the females never get dark types is because Misty only caught water types and May and Dawn only get cute pokemon. There just aren't any cute dark types. Now if Absol or Sneasle got a pre-evolution that was cute or something, I could see one of the girls getting that.
Team Rocket can get dark types too, their teams are pretty limited too. Jessie always has Wobuffett, a snake and a flying bug, James always has his annoying baby pokemon and a grass type. If the 5th gen. had a dark type snake in it I'm sure Jessie would replace Seviper for it. The reason Cacnea didn't evolve was because Harley had Cacturne.