Well, I don't have a good reason why they would be capped at 9 hours but it might be a combination of a few things. First it would look odd to teach more than a full time person. Second, adjuncts often teach to an expertise and there might not be enough classes in that area to offer more. Third, three classes is sometimes a healthy portion of classes in one subject and students should have some variety in professors. There are probably other administrative reasons but those are the ones from the teaching side I can imagine. But I don't think allowing them to teach more will help the problem. 9 credits implies about 20 hours a week including grading and answering emails and prep. So even 18 credits per semester twice a years is $60k and one would be hard pressed to live well on that especially if one had the qualifications to be a professor. Except in some fields, you can often find a job making twice adjunct pay easily. I just do not see being an adjunct exclusively as any way to try to live in NYC, unless you have money already and do it for other reasons.