With Active Directory Certificate Services (AD CS), each logical Certification Authority generally requires its own Windows Server instance. Depending on the size and structure of the organization, it can be useful to separate Certification Authorities by purpose and scope.
Many organizations also operate multiple Active Directory environments and CA hierarchies, which can result in a larger number of CA servers that must be managed, hardened, patched, maintained, and financed on an ongoing basis.
Modern PKI platforms can support multiple Certification Authorities within a consolidated platform, helping reduce the number of required server instances, simplify administration, and lower overall operating costs.