You can enable the vCenter Server to distribute the virtual machines across multiple ESXi hosts based on their resource requirements. To achieve this you need to enable vSphere Distributed Resource Scheduler (DRS) on the cluster.
The following procedure explains how to enable DRS on a cluster:
From the vCenter's inventory home navigate to the Hosts and Clusters view.
Select the cluster and navigate to Manage | Settings | vSphere DRS and then click on Edit….
On the Edit Cluster Settings window, select the Turn ON vSphere DRS checkbox. Leave DRS Automation, Power Management, and Advanced Options at their defaults for now.
Click on OK to enable DRS.
The cluster's settings page should now show DRS enabled. It should read vSphere DRS is Turned ON.