This question can't be answered per se; the bandwidth needs of e.g. (a) a self-contained office that has a file server in-house, and (b) a site that constantly moves large image files in and out the building, makes extensive use of cloud storage, and has a bunch of VPNs to remote sites, can easily be two orders of magnitude different, even though both sites have the same number of people.
What you can, and should, do is monitor like a hawk. Make sure that you're using equipment that lets you monitor bandwidth and latency, both to your next-hop-route outbound, and to some external resources that you need to monitor, constantly. Data every five minutes is a minimum; every 60s is better.
If you have particular sensitivity to certain network resources (latency to a remote asterisk server? bandwidth usage to/from your cloud storage provider? throughput to the VPN peer in Spain?) make sure that you're also monitoring those particular parameters.
Graphs and data averaging should be happening automatically, so if someone wants to know how the connection is doing, you can answer that question instantly.
Over a period as short as a couple of weeks, you should be able to build up a picture of what you're using your internet connection for, and how well it's dealing with the demand. From there, you'll have a much better idea what you're likely to need in the both medium- and longer-terms.