You can launch as many subkernels as you want (and as your license allows) using
LaunchKernels. Mathematica launches as many as the number of CPU cores by default, but you can specify the exact number you want, and you can launch more.
Using more subkernels than the number of CPU cores you have will typically lead to no performance improvement at all. Sometimes it leads to worse performance. In rare cases, it may be worth doing with a hyperthreading CPU. I strongly recommend you do some benchmarks to verify that you do actually gain performance before launching more subkernels than the number of CPU cores.