Figures such as 10,000 ATMs per annum used to be bandied around but DAP doesn't use single triggers any more (if they ever did). It's all about traffic density and complexity so it's more risk-based.
As a guide to future change, I think the last airports to lose their CAS were Prestwick and Southend. They were running at about 3000 and 5000 ATMs a year respectively when they lost their CAS in 1992. But I think DAP would have a much harder time withdrawing CAS these days because they'd have to demonstrate that the risk level for any remaining CAT flights was acceptable.
Doncaster has picked up a little since last year but Durham is now below 6,000 ATMs and continuing to decline....
NS