There is no limit - the only issue is if your questions are marked as bad. If you ask questions that are answerable and work on StackOveflow, you can ask as many as you like.

If they started banning users for asking questions or even for asking poor (read extremely poor) questions, then the overall traffic would reduce significantly.

I wish. We've been doing this for 5-6 years now, and it hasn't. Quite honestly, if we could get rid of the worst 20% of questions instantly, life would be a lot better for the remaining 80%... But, identifying those 20% when often SO is their only option is... Brutally hard. I've seen the same person try to post variations on the same question dozens of times before finally getting around the quality checks... Only to have it sink like a rock because of course it was still unintelligible after all that.