The Advanced Runtime Info box might display a smaller number of available slots in the cluster than you expect.
When you select the Host Failures Cluster Tolerates admission control policy, view the Advanced Runtime Info pane that appears in the vSphere HA section of the cluster's Monitor tab in the vSphere Web Client. This pane displays information about the cluster, including the number of slots available to power on additional virtual machines in the cluster. This number might be smaller than expected under certain conditions.
Slot size is calculated using the largest reservations plus the memory overhead of any powered on virtual machines in the cluster. However, vSphere HA admission control considers only the resources on a host that are available for virtual machines. This amount is less than the total amount of physical resources on the host, because there is some overhead.
Reduce the virtual machine reservations if possible, use vSphere HA advanced options to reduce the slot size, or use a different admission control policy.