<<From personal experience, very few. Never heard another aircraft on frequency in Europe declare a TCAS RA.>>
I'm surprised at that, Mike. I've certainly seen a fair number in the London TMA whilst working as a Heathrow radar controller - the majority neither the fault of ATC nor the crews. Whether TCAS has been sufficiently modded to reduce them I don't know, but they tended to occur with traffic descending fast into holding patterns, or climbing fast under already level traffic. It's quite scary when you have a holding pattern full and someone says "TCAS RA; we're climbing"!