A 2019 study of 5 European airports in 2019 had no-show rates of 14.4%: https://www.ozion-airport.com/product/comparative-analysis-n...

However, my understanding is that airlines have much more sophisticated per-flight and per-passenger models that calculate the predicted no-show factor based on the historical rates for that particular route (e.g. you're more likely to get more no-shows in business class flying from NYC to SF compared to holiday travelers with a reservation on the Florida Keys)

I think you'll have to pay a team millions to figure that out, it is unlikely to be a static rate but rather decided based on multiple traits like time of year, time of flight, distance of flight, cost of ticket, etc.