Yes, You can use Single Adsense Account for Multiple sites. BUT GOOGLE ADSENSE GUIDELINES SHOULD BE FOLLOWED STRICTLY ON ALL SITES. Otherwise, your account will be banned. I have pasted here text from Google Adsense Policy Page and the matter which describes policy for using single adsense account on multiple sites is highlighted in bold.
There's no need to open another account or notify us when you place
your ad code on a new site. Once your account is approved, whenever you
place your AdSense code on a new page, we'll automatically associate
that new site with your account.
Generate the AdSense code as you did for your original site, and add it to whichever page you'd like — making sure, of course, that it complies with our program policies. You don't need our team to officially review your new site beforehand, but please use your best judgment to make sure that your site content and ad implementation are compliant. (Our policy explanations might help.) We regularly review sites displaying our ads, and we'll let you know if we find any issues.