Quantity or Quality

People tend to think that more number of websites and blogs would help them earn more. This is a misconception where many a times publishers instead of increasing their earnings end up decreasing their earnings as well as visitors due to decrease in quality.
" Don't try to eat more than what yo can chew."
It is not wrong to expand your source of earnings through Adsense but it is necessary to identify the best potential source of revenue and concentrate on that. It is not the number of sites which will increase your revenue but the quality of those sites which will do it.
For beginners it is recommended that they aim of developing their site and enrich their site with original, quality content. Prefer to write on topics in which you have expertise or are passionate about. Design the website keeping the general webmaster guidelines and more importantly your users in mind.
During this make full use of webmaster tools, Google Analytics and Search Engine Optimization Techniques. And of course if you have an adsense account then experiment with ads and placements which best monetize your site.
One more thing which you should always keep in mind that income through online advertising like adsense doesn't starts overnight but requires regular hard work and patience because it does take sometime before your regular earnings reach your desired level.