I was tempted to say "just the right ammount", but that really isn't how I feel. You could argue that there are too many dvd's coming out, and I would agree with that; however I don't feel its the number of DVD's coming out every week, rather the format in which they are released.
Rather than seeing four disks coming out at a time, covering 3-5 episodes of four seperate series, I'd rather see more disks for half the number of series, so individual volumes covering 6-10 episodes of just two series... but for the same price.
This cannot happen I know, but it's the only thing that would encourage me to fork out twenty quid each week on new releases.