The RNR headcount is drawn from the requirement by HQ NAVY to provide X number of augmentees in various roles to support a fully mobilised RN conducting operations, plus NATO requirements plus a margin.
As the RN has been cut, and as some RN defence assumptions have been changed, it is likely that the RNR headcount will be reduced to meet the new defence assumptions. I wouldnt worry too much though, there is plenty of deadwood in the system who will go before active members do (or one would hope that to happen!)
There is also a wider review into the Reserves going on, which will report next year. Too early to say what will happen, but it appears that No10 is pro reservist though, so dont worry too much - yet!