No matter how many domains a company has they can use the same tracking server for all the domain which is third party cookie implementation.
But if you want the cookies to be created on the domain and want First Party Cookie Implementation then there can be two approaches to handle the situation.
Suppose : They you have 10+ different domain
Where we have to configure 10+ separate tracking server values for respective domains.
Tracking server secure value like smetrics.abc.com for all the 10+ domains – in this case, except for the main domain, the rest would become the friendly third party implementation.
Friendly 3rd-party cookies: Used primarily by organizations with multiple domains that want to use a single visitor ID across all tracked sites. For example, an organization that owns both example.com and example.net could store the cookie on metrics.example.com
As per https://docs.adobe.com/content/help/en/id-service/using/reference/ecid-library-methods.html “the reason for the shift of ECID to CNAME implementations is for persistent visitor identification, not multi-domain tracking”
Option 1 is the best choice here –
It allows for more persistent visitor identification (with option 2, we don’t get a 1st party cookie set server-side)
If there are links that lead for instance from domain X to domain Y, then use appendVisitorIDsTo method for cross-domain tracking
Option 2 doesn’t provide much benefit
First Party Cookie implementation is very much required to mitigate the impact of ITP 2.x