I have worked on many (30+) webinars both as a marketer generating the audience and as a speaker over the last four years. Based on my experience, I find that generally you want two or three speakers depending on the length of the webinar, speaker level of expertise and involvement of the host/MC.
When you have only one person, if the webinar platform is not truly interactive it can be a lot like talking to the wall for the speaker. Also, even if the platform is interactive and allows for the audience to post questions, comments, etc., it is difficult to give the presentation while reading and answering questions from the online audience at the same time (not during a Q&A portion). If the speaker is typing in an answer to a question in the chat room, while speaking on a topic, usually you get some dead air there. True, there are some speakers that attempt this, but only a few can do it and still maintain the flow of the presentation. Finally if you have only one speaker they need to be really well known to be a draw for the audience. Typically you will reach more people with two or more speakers, especially if they assist in promotion.
Often having a two or three speakers allows for the webinar to be more conversational and engaging for the audience, especially for longer format pieces. If the webinar is short, 20 to 30 minutes, then you really don't need more than one person, however for longer format shows it can be difficult to maintain the audience’s interest over time or simply to continuously talk for 80-90 minutes straight without a break. Plus if you have two people speaking, one can monitor and respond to chat or online questions submitted, while the other continues the presentation. Two or more speakers and/or a host that work well together can take a more fluid approach to their presentation, using audience questions to pass topics back and forth, and set up transitions from one topic to another, etc.
As Jeremy mentioned above, multiple speakers also allow different viewpoints to be presented and developed during the program, without self-contradiction. However having too many speakers can make it difficult to manage the content production and timing during the webinar, also the audience may become confused trying to follow four or more people during a presentation that uses slides instead of showing the speakers – that works better when the panel is live.
I hope that answers your question, feel free to contact me if you have more questions.