A: Many organizations opt to use a KMS internally, which is fairly easy to install. You set up a KMS key and clients, then activate with the internal KMS server instead of Microsoft. This is only used for the initial activation—clients need to check in every 180 days, and they'll actually try and reactivate every seven days, which then resets the 180 day counter. (So even if the KMS server is unavailable for a while, it's unlikely you'll have problems with your clients, because they should have plenty of time left on their activations). If your KMS server fails, it's very easy to stand up a new KMS server in its place.
You might still want multiple KMS servers, either for geographic distribution (which is the most common reason), or simple scalability. The KMS key you receive can be used on up to six KMS servers in your organization, per this Microsoft article. However, if you contact the Microsoft activation center, they can enable additional activations for your KMS key.