This depends on plethora of factors: like the industry to which the webinar is pertaining to, software and technical industry has the highest registrations and participation on an average. Also the time of webinar, day of the webinar hosting, The presenter (well known and professionally acclaimed attracts more registrations and participations), length of. The webinar (this forms a bell curve with peak at 55-60 minutes) etc. Coming to statistics of targeted audience to registrations it could be as low as an average of 30% for a cold audience (which don’t know your brand and have never engaged with you) to as high as an average of 60% for a known and hot audience list.
I agree with the previous comment that this could depend on many factors.
I'd encourage you to start benchmarking your own performance, if you don't have historical data you can pull from. You may decide to look as high-level as the webinar channel, or focus in on the types, audiences, or topics of webinars.
It's important to also look at the source driving to registrations, and looking at all of the other related metrics around attendance and new names, etc.