32Gb needed according to this: https://docs.vmware.com/en/VMware-vSphere/7.0/com.vmware.esxi.upgrade.doc/GUID-DEB8086A-306B-4239-BF...
Before Upgrade please note the below points.
VMware Valid Subscription
Hardware Comparability to verify its support ESXi 7.0 U3.
Existing VMware Infrastructure Health Check.
Upgrade the Firmware in Physical Host.
Upgrade the vCenter
Upgrade the ESXi Host
Upgrade the VMware Toolbar.