What percentage of registered participants eventually show up at conferences?
I am afraid that, especially for a small meeting, the variation of how many do not show up will be too large to be useful. For example, if I am organizing a 50-person event I would expect that anything from 2 to 10 would be a "normal" number of no-shows. That is, if you really can't have more than, say, 50 participants you can't really safely overbook at all. However, note that oftentimes a few participants more don't really hurt, but of course this depends on your infrastructure and plans for the meeting.

Some further considerations:


Generally, free events have a very large number of no-shows (sometimes in the 50%+ range, from what I have heard) while paid ones do not. This has led to some meetings I know introducing a nominal attendance fee, just to make sure that people think a little before signing up. Some other meetings have also tried to levy a fee only if a registered participant does not show up, but I am not sure how well this works and if it is worth the administrative and inter-personal hassle.
Note that no-shows are often not independent events, statistically speaking. For example, if you have a bunch of participants from the same group / team and they have an unexpected important deadline come up, they all will cancel at the same time.
If you have a very small meeting and you know (some) of your participants well you can attempt to use this to make a stab at estimating no-shows. For instance, I once planned an event for about 30 people, and I knew that some of the senior professors who initially expressed interest where &lt;50% to actually come. I also knew that for some fraction of the industry participants an important client meeting may come up making them cancel last-minute. Based on this information I decided that 35 registrations for a 30-person meeting was fairly safe (we ended up being 28).