How many fastener can an aircraft lose?
There is no straight answer as it depends on the individual fastener and specific aircraft in question. Pressurized aircraft create a whole new set of issues.  

In some cases you can lose a whole bunch of them and even lose a full panel without issue as was the case on Delta  2412. While in other cases single bolts or fasteners are far more critical. Fasteners on stationary parts wont create the same imbalance issues as say, losing a fastener from a prop spinner or the such. Fasteners coming loose and being ingested into engines or other critical components can be an issue which is similar to the issue with the Concorde incident.