An interesting pre-print of a paper that will appear in the professional journal Astronomy & Astrophysics is available online today (note the full paper can be downloaded as PDF from the page I have linked)
Some of the authors are well-known amateur astronomers and the paper covers recent impact events on Jupiter (Anthony Wesley has spoken at BAA meetings). The paper considers the size of the impactors and the frequency of occurrence. The authors point out that amateur observers can play an even greater role in detecting and observing these events with improving camera technology coming onto the market.
We live in exciting times for amateur planetary observation! If you want to hear more about the cutting edge of amateur imaging, in this case of Neptune, have a look at the talk by Mike Foulkes given at the last BAA meeting.
Certainly more have been seen than that article suggests. (Wikipedia lists 7 not including Shoemaker Levy). Marc Delcroix coordinates an amateur programme for Jupiter and Saturn impacts. Based on their coverage they estimate a rate for potentially detectable Jupiter impacts of ~6/year