A process for service chain instance pool sizing receives service requests for processing in a 5G network by a service function chain, of the 5G network, providing network function(s) to process the service requests. The method determines an average number of service requests to be processed, and determines a number of service chain instances to exist in a pool of service chain instances, of the service function chain, as a composite of (i) a number of instances to process service requests having arrived and making an initial relay through the service function chain and (ii) a number of instances to process service requests that were re-introduced after having previously arrived. The method additionally indicates the number of service chain instances to exist in the pool to a component responsible for controlling the number of service chain instances in the pool.