Jimmy,
Thanks for reaching out! You don’t technically need to have a license key on each site, as long as you have one on the main site. That will give you access to support and updates. However, without a license key on the other sites you will likely need to manually download and install updates for those sites. If manually updating would be problematic for you, then you’d want to buy a larger license package.
Hopefully that answers your questions, but let me know if you need further help. Thanks! 🙂
-Casey-