You can have has many has you want. If i have more than a ads block i give them a diferent channel so i can know wich one is the best and try to improve the others
Well, you can only have so many ads on a page, but each one can be a different channel, sure. To know where your money is coming from, label them so you know exactly what page they're on & what type of ad they are. Adsense allows you to combine channels now, so you can have have some channels relate to specific types of pages, & some relating to the type/size of ad and/or its placement on the page. On a Britney Spears fan site (just as an example), you could have ads that are labeled 'britney-vertical160x600-txt.' That will tell you what page it's on, size/shape of ad, & whether it's text or image.
firstly when you add AdSense to your site you should have different chanels for each ad this way you will know which one performs well and you can optimize the ads after that you can delete them all