In a high-density Remote Desktop Services (RDS) environment, multiple users share the resources of a single server virtual machine. This makes the workload highly sensitive to resource contention, especially CPU. A key Cisco and virtualization best practice is to avoid or be very conservative with CPU over-commitment. Assigning more virtual CPUs (vCPUs) to virtual machines on a host than there are physical cores can lead to high CPU Ready time. This metric represents the time a VM is ready to run but must wait for a physical CPU to become available. High CPU Ready time directly results in poor application performance and a degraded user experience, which is critical to avoid in VDI and RDS solutions.

A. The number of users per host is highly dependent on the specific hardware, user workload, and applications, not a fixed number like 20.

C. Changing the virtual host disk queue depth is an advanced storage performance tuning task, not a primary best practice for initial compute sizing in an RDS solution.

D. An RDS server virtual machine must serve many concurrent user sessions and therefore explicitly requires multiple vCPUs to provide adequate performance. This advice applies to single-user VDI, not RDS hosts.

1. Cisco Validated Design (CVD): FlexPod Datacenter with VMware Horizon 7 on VMware vSphere 6.7 Design Guide. In the "Sizing the Solution" -> "CPU" section, the guide states, "CPU oversubscription should be approached with caution. The ratio of virtual CPUs to physical cores can have a significant impact on performance." This directly supports the principle of carefully managing or avoiding CPU over-commitment.

2. VMware Documentation: Best Practices for Published Applications and Desktops in VMware Horizon 7 and Horizon Apps. In the "RDSH Server VM Configuration" section, it advises, "Monitor CPU usage and CPU ready time. If CPU ready time is consistently high (greater than 5 percent), this indicates that the host is overcommitted." This highlights the negative impact of over-committing CPU resources, reinforcing the answer. The same document also recommends sizing RDS Host VMs with 4 or 8 vCPUs, directly contradicting option D for this scenario.

3. NetApp Technical Report (TR-4823): FlexPod Datacenter with VMware Horizon 7.8 and VMware vSphere 6.7u2. The "CPU Sizing" section emphasizes monitoring CPU Ready time and states that "CPU oversubscription is a key factor in determining the number of users that can be supported on a given hardware platform." It advises a conservative approach to ensure a good user experience, which aligns with avoiding aggressive over-commitment.