I want a specification for a server that can act as a Domain Controller and Active Directory for 400 Windows clients.Look Here:
http://techgenix.com/active-directory-insights-part5/
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Active Directory Insights (Part 5) - Domain controller hardware sizing
Mitch Tulloch Posted On September 5, 2015
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If you would like to read the other PARTS in this article series please go to:

    Active Directory Insights (Part 1): Configuring DNS on domain controllers
    Active Directory Insights (Part 2): Digging into Trusts
    Active Directory Insights (Part 3): Re-examining read-only domain controllers
    Active Directory Insights (Part 4): More on read-only domain controllers
    Active Directory Insights (Part 6): Domain controllers and NIC teaming
    Active Directory Insights (Part 7) - More on using virtual domain controllers
    Active Directory Insights (Part - Virtual domain controllers and disaster recovery
    Active Directory Insights (Part 9) - Automating user account provisioning
    Active Directory Insights (Part 10) - DHCP and domain controllers
    Active Directory Insights (Part 11) - sIDHistory
    Active Directory Insights (Part 12) - MaxTokenSize
    Active Directory Insights (Part 13) - Digging into the GLOBAL Catalog
    Active Directory Insights (Part 14) - More about the Global Catalog

Introduction
Domain controllers are the heart of a Windows Server environment. They're the KEYS to the kingdom, and because of this they need not only to be secure but also reliable and always available. Having at least two domain controllers in each domain (and SOMETIMES at each site) is essential to ensure directory services are always available for users so they can be authenticated and authorized for accessing resources. Physical domain controllers also need to have system hardware that has a high degree of reliability and resistance to the different kinds of hardware failures that server systems can be prone to. And virtualized domain controllers need to be run on host machines that have hardware with similar reliability requirements.