In a physical network device, there is port number/port ID such as Ethernet1/1, GigabitEthernet1/1.
You can think port ID is like a port number but within a VDS.
In vSphere 6.5 for example, maximum number of VDS ports are 60,000 https://www.vmware.com/pdf/vsphere6/r65/vsphere-65-configuration-maximums.pdf
You can view the port IDs by selecting VDS > Manage > Ports

Everytime you create a PortGroup, it will consume Port ID by default is 8 Ports but this can be changed.
For example I create a new PortGroup name "New PortGroup" and I put the number of ports: 2 (by default is 8)

2 new Port ID will be allocated to that New PortGroup, in below example Port ID 130 & 131

Note: when we create a PortGroup, there is an option called "Port allocation" and this can be set to elastic or fixed
Elastic: The default number of ports is eight. When all ports are assigned, a new set of eight ports is created. This is the default.
Fixed: The default number of ports is set to eight. No additional ports are created when all ports are assigned.
Hope this helps