HA Admission Control is a function that will guarantee that enough resources are avaliable for HA after "some" failure
You can set HA Admission Control based on percentages of resrouces, you can use dedicated failover host or slot-policy.
Usually what I see is the percentage reservation, meaning that you won't be able to alocate 100% of your cluster capacity but certain percantage will be "reserverd" for failover.
If you have two hosts, then the HA Admission Control should be 50% (in ideal case), meaning that if one of your host will go down you have anough free resources to utilize VMs running on crashed hosts. If you have 5 hosts, then 20% should be reservers (and so on).