Uma carga de 10 é razoável neste caso. A regra básica é que você quer que sua média de carga seja menor que o número total de núcleos. A razão pela qual você parece ter o dobro da quantidade de núcleos é devido ao hyper-threading . Aqui está um trecho do artigo da Wikipédia:
For each processor core that is physically present, the operating system addresses two virtual or logical cores, and shares the workload between them when possible. The main function of hyper-threading is to increase the number of independent instructions in the pipeline; it takes advantage of superscalar architecture, in which multiple instructions operate on separate data in parallel. With HTT, one physical core appears as two processors to the operating system, which can use each core to schedule two processes at once. In addition, two or more processes can use the same resources: if resources for one process are not available, then another process can continue if its resources are available.