How many domains and domain controllers were in your infrastructure?

In my infrastructure, there were usually between 1 and 3 domains, depending on the scale and structure of the company. For each domain, several domain controllers (DCs) were typically deployed to ensure fault tolerance and load balancing — usually from 2 to 5 controllers per domain. For example, in one project, there was a primary domain with 3 domain controllers located in different data centers for geo-redundancy.