Yes You can keep as many site you want. but as per Google Policy, one persion can hold one adsense account.
Yes, you can use more websites or blogsites for Google adsense. Just like what I did, I put my adsense code in 4 blogsites of mine. But I only have 1 account in adsense.
Yes you can use one adsense account ads to any websites those are match with adsense policies. but need to add sites to adsense account under URL channel.
As long your site is according to adsense accounts, then you can put adsense on as many sites as you want.
make sure that no body else is using you adsense code in an adult website (or your account will get suspended). try to make changes in you Adsense account to specify the domaine that you use for your Ads !
Never apply for a second account, you only need one, although some people on here claim to have 5 or 6 - WHY???? when you only need one. Applying for a second one will get your existing one banned, along with all your sites, and it's against Googles Adsense TOS that people first sign up to. A fact strangely forgotten by many people who get banned......