Let me ask another question to help assess the risk... In the intermediate level, on average how many cars sustain serious damage per track day? How many cars are running in this group on a typical track day?
To be honest, the 'beginner' and 'intermediate' level HPDE drivers (weekend track drivers), are generally the ones that will have the most offs/incidences.
A) n00bs tend to be really cautious, or overly confident/show off. Both are dangerous in itself. One being they're totally out of their comfort zone, and the latter, simply driving beyond their skill level.
B) At the intermediate level, most drivers have not reached 8 or even 9/10th or their driving ability yet, but are working on getting there. Thus, they will tend to push their limits, which is a good thing - its the only way to improve, but that is also when some people will realize that they just can't handle the car when its just a little bit faster, or they just can't think that fast, or simply not ready yet.
Experienced drivers have incidences too, don't get me wrong, it happens to everyone (me too!) but its how they are handled at the moment it occurs that usually determines the outcome - minor or really bad. That only comes with experience. Kind of a catch 22 I suppose.
But back to your question, some track days will have 0 'incidences'. By "incidences", I mean no fender benders, no major offs that involved a collision, etc (keep in mind there's typically no passing at HPDEs except in designated sections of the track, so any 'collisions' are from hitting barriers or immovable objects, not one vehicle with another)
Other days can have 1 or 2 (or more!) minor incidents, ie: offs that didn't results in a collision, but ended breaking suspension parts or something that ended up in a flat tow.
I wouldn't be surprised though that if there was even one CLOSE CALL, the track officials would stop everything and have a serious conversion with everyone immediately and discuss (revisit) the safety and important of awareness while being on the track before something (else) happens.
But, on the average, i'd say that at any given HPDE, at least ONE person will break and require a tow truck or crash with body/suspension/engine damage of some sort and may or may not require a tow home. This is with participants as few as 30-40 up to 60-80 on a given day.