Erm, yes, if one of those sites doesn't comply with the adsense ToS ... or you click your own ads ... etc etc
You can have adsense on an unlimited number of sites. You're only restricted to so many adsense ads per page though. I recommend you check out the program policies -> https://www.google.com/adsense/supp...orefrontEN_v2_ProgramPoliciesLink&medium=link
no, you wouldnt get banned because it is allow to put adsense ads on multi-website as long as all the website is not against adsense TOS