Really I don't think there's enough material in the comics to support a very long series, I think they would have to pad it out somewhat if they wanted anything more than a season or two of episodes.
My guess is that they will do what other properties have done, use the original text as a jumping-off point and spin characters and stories out of that in more detail.
The danger I think is that Sin City is very deliberately a pastiche with very over-the-top noir elements and a fairly caricatured approach to its characters. I think adding more depth risks losing something of the simplicity that makes it attractive in the first place.
We'll see I guess.