Curb your Enthusiasm is produced by and aired on HBO in America. Cable shows (on HBO, Showtime, AMC, USA, TNT) typically have 10-12 episodes per season. On the other hand, the network shows (on NBC, CBS, ABC, FOX) are the ones that can have 20-26 episodes per season. Why this happens is not set in stone... the cable channels are usually pay channels, have lower consumer penetration in general, have lower advertising costs, lower budgets and hence fewer number of episodes per season. Although the lower budget part isn't strictly true... specifically HBO and Showtime shows have very high production values. It's just something that is.
The big networks are opposite and hence can afford more costly seasons... although a new trend is starting to show up recently. Now even some network shows are getting cut down to 16-18 episode seasons (eg. LOST, the next season of Heroes etc.) All this is an attempt to bring down the costs of creating the shows.
Also, Curb your Enthusiasm has already had 6 seasons, and season 7 is in production right now