BGP is the protocol used to talk between networks. It is the protocol of the Internet. To connect your network to other networks you will likely use BGP or have your uplink provide you with a gateway IP address. BGP is not needed to connect to the Internet or to other networks, but it is common.
A single port can be used to connect to multiple ports on another network. It can even be used to connect to multiple different networks, all running BGP. So the answer it yes, it is possible to have multiple BGP sessions running on a single port. The only real requirement to do this is to ensure you have a different neighbor IP address for each of the BGP sessions you will be running. Each of your BGP peers must have a unique IP address.
Running many BGP sessions on a single port is actually very common. This is exactly the setup that Internet Exchange Points (IXPs) provide. They allow your equipment to connect to their switch, which has multiple connections from different networks. You can then create multiple BGP sessions through this switch to all the different networks on the switch.
You can also have multiple BGP sessions with a single network. For example, your port is connected to a switch and then that switch goes to two different ports on your uplink network.
The reverse is also possible. If you have two different routers and want each of them to run BGP with a single neighbor IP address, that is totally possible.
Sometimes you will want an unique VLAN for each different BGP session. But as long as there are different neighbor IP addresses, multiple BGP sessions over one port are possible.
