Yes, you can. One code can be used for dozen sites, there is no binding.
This means that if, for example, you have two websites, you can use the same code for both of them.
It is rather for convenience and order. If you create spots for each website, you can see statistics for each website/spot, otherwise, you will see it as a single website.
If you have 2-3 not similar websites with HQ traffic, it is better to create spots for each site, but if there are several decades-hundreds similar sites, it's normal to use 1 spot for all of them.
Nevertheless, we recommend verifying each of your top Websites and creating Ad Tag for each one as Direct Advertisers would be able to run a different bidding strategy for each one.
Also, it's recommended to verify each website within Ad Networks as it would work the same way.