Adsense Users- new site owner

Can different adsense accounts be associated with the same adsense code snippets? Each adsense account tracks each adsense unit, and “knows” that it created it, right?

An AdSense Ad Unit is associated with an AdSense account via the publisher ID in the code snippet.

Do you mean to add more publisher IDs to one AdSense code snippet?
That would not work and is not allowed otherwise each click would be payed twice or more which would be cheating the advertiser paying for the click.

You are also not allowed to have more then 2 accounts (one personal and one business account).

Not just the ‘publisher id’ but the ‘ad slot’, with the snippet generated in the account. For a single user only and single values for these.

Wondering if there could be any issues when a code snippet wasn’t generated in the account that is being tracked. Like when a site is sold. Analytics UA is changed out but what about Adsense?

If you buy a website you should make sure you replace the old owners AdSense code with yours otherwise he will earn your money.
The Ad Slot stores information like the color schema, the fonts, the corners and channels attached to this ad unit.

It’s very important to move the data onto a new ad account, otherwise you will not receive the money made.