HA Admission Control ensures you have sufficient resources to restart your virtual machines.
HA Admission Control uses reservations or overhead to calculate how many VMs it can restart. And that is where your problem might be, if you have NO reservations set then the amount of VMs you can power-on before HA complains is high... extremely high potentially.
As Frank said, HA is not a resource management solution. It is an availability solution, ensuring VMs can be restarted.
So if you have HA Admission Control enabled you guarantee that VMs can be restarted, but not that they get the resources they potentially require. For Resource Management and Distribution you will need to look in to VC Ops / DRS.
I wrote about a thousand articles on admission control: