how many sites should i put adsense in it? is they check the sites to know if it legal or not to use ?
You don't need your sites reviewed by google again if you have been approved, but DO keep them in legal sites as they can find out. Doesn't matter how many sites you should place in adsense. How many sites do you have? I place them in all of my sites EXCEPT my portfolio sites (as it makes them look unprofessional)
The sites you place your Adsense ads on need to adhere to the Adsense TOS it's really that simple. Placing Adsense on poor quality sites (Even though they are not a violation of the TOS) can LOWER your overall earnings as Google applies Smart Pricing to Accounts not to sites or pages.
Can any one please explain me all about smart pricing? and why google lower earnings? any more details?
You can place google ads on all your websites and you don't need any approval for each site. Just make sure that every website follow the TOS.
With Smart Pricing, Google may pay you higher than others for the same clicked ads if you have high quality sites/pages in which your adsense codes placed.