Directly from your Adsense Welcome email :)
- You can add the AdSense code to a new page or site that complies with our program policies at any time. There's no need to inform us or
apply for a new account when you do. Please make sure our crawler can
access any webpage you place ad code on: http://goo.gl/NyUEy
The Google AdSense Team
Google posts more information on how to add more sites to the account here (I've copied/pasted below for conveinance) - https://support.google.com/adsense/answer/65063?ctx=as2&rd=1
How to add more sites to the account
There's no need to open another account or notify us when you place ad
code on a new site. Once your account is approved, whenever you place
your AdSense code on a new page, we'll automatically associate that
new site with your account.
Just generate the AdSense code as you did for your original site, and
add it to whichever page you'd like -- making sure, of course, that it
complies with our program policies. You don't need our team to
officially review your site beforehand, but please use your best
judgment to make sure that your site content and ad implementation are
compliant. (Our policy explanations might help.) We regularly review
sites displaying our ads, and we'll let you know if we find any
Note that if you're using the Authorized Sites feature, you'll need to
add the new URL to your Allowed Sites list, or you won't receive any
earnings for that URL.
We look forward to helping you monetize your new site!
So you do not need to inform Adsense of new sites/pages you wish to display your Adsense Code on. Though I'd definitely recommend using "Custom Channels" to allow for easier tracking of separate sites/pages/adcodes.
As well as having multiple Adsense Accounts is a violation unless the circumstances are within those in which Google states is allowed - https://support.google.com/adsense/answer/9729?hl=en