How many domain controllers should I have?

Unless you have some application in your environment that generates high kerberos traffic, then technically a single server with Microsoft recommended hardware specs [or VM provisioning] should would just fine. However, you should have at least two domain controllers for redundancy, and preferably one domain controller in each site.
For More Information Please Refer:
https://social.technet.microsoft.com/forums/windowsserver/en-US/991d4f68-5178-4c9a-8b7d-8f2b5f53867e/how-many-domain-controllers-are-recommended
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