This subject is very large, but I will try to cover some key points.
The bandwidth of the link to the ISP is 100 Mbps. That means that your router can send 100 Mb to the ISP's router each second, and the ISP's router can send the same to your router.
You must consider the traffic direction.
For outbound traffic, the bandwidth for each computer is whatever the link speed from the computer to the next device is. For example, your computers could have 1000 Mbps links to the access switches.
The access switches to which the computers connect will have links to some type of aggregation, and those links will have a bandwidth associated with them. The aggregate access bandwidth compared to the uplink bandwidth will give you an oversubscription ratio. For example, if you have a switch with twenty-four 1 Gbps access ports, and a 1 Gbps uplink, you have a 24:1 oversubscription ratio.
You can also calculate an oversubscription ratio to the router, and to the ISP's router.
It is not as simple as dividing the bandwidth of the oversubscribed link between all the computers. For the example of the 24-port switch above, one computer could send 1 Gbps to the aggregation switch if it is the only one sending. If all 24 computers are trying to send 1 Gbps at the same time, a lot of traffic will be dropped. The frames going to the aggregation switch will be sent on a first-come, first-served basis. Switches have very, very small buffers, so the vast majority of the traffic will be dropped.
How that affects things depends on what is being sent. For example, UDP will be lost, but TCP will request lost segments to be resent.
For inbound traffic, it is unlikely that any links are oversubscribed, and you usually have much more inbound than outbound traffic. You really don't have much control over the traffic from the Internet to your network. If a single request from a computer on your network starts another computer on the Internet sending 1 Gbps of traffic to your network, it can basically monopolize your inbound ISP connection. Someone wanting to disrupt you network can try to overload your incoming network connection in a DoS (Denial of Service) attack.
There is QoS to try to introduce your vision of fairness to your network. For example, you can prioritize real-time traffic, e.g. VoIP. You can't really do a lot about the traffic coming into your network from the Internet because you don't control the Internet.
