During my Trainer Talk Live presentation about live online sessions, Sheridan Webb asked about the bandwidth needs for delivering webinars and also with a tweet.

Obviously the more the better and it depends on the platform and which bits you use, such as screen share or webcam. 

An article on logmein states this:

In general, you can estimate the amount of bandwidth a session will require using these data-usage averages:

• Webcam sharing (HDFaces) = 700 Kbps (0.7 Mbps)
• Screen sharing = 40 Kbps (0.04 Mbps)
• Computer audio (VoIP) = 40 Kbps (0.04 Mbps)

A Zendesk article started this:

The optimum speed for hosts and attendees who want to sharemicrophones, web cams, desk tops, and presentations is 10Mbps+. If possible, a wired connection is much more stable and will help prevent dropped connections and interference.

And this Zoom article has more detail too.