Hello... I'm a complete newbee with adsense. I have different niches for each site that I have, my question is if I can use the same adsense code for each website?
Yes you use the same code. In the adsense control panel you create channels to see where your clicks is coming from.
Yes, you can use it for multiple sites. Make sure where you float your account because if you do it on a site that is violating adsense TOS then your pub id might get banned.