I would say that there are maybe two or three single engine landings a month at least.
One thing to bear in mind though is that if you have two engines and you have a problem indication on one of them you can do a single engine approach and not risk using a potentially dodgy engine.
On Tornado a large number of single engine approaches were for transitory engine vibration which on further investigation turned out to be the detection system inducing a signal and not a problem with the engine at all. Lots of single engine approaches when not required but would you want to take the risk of an engine failing on you at a critical time?
Happy to say the system was modified so the number of spurious VIB captions has dropped.