End Google Ads 201810 - BS.net 01 --> Senario is like this;

I have a wcf service, this service sum two number. So "sum" class run on the server side. Client use following code;

Sum sum = Sum.Instance;
sum.AddParameter(1);
sum.AddParameter(2);
int result = sum.Calculate();

This service is run corectly when if at the same time ten client call this service and add different parameter? My main question is singleton pattern using is it wrong in this senario? Because all client use same instance.