The VM has only a single CPU configured, so there's no option to make it a multi-core CPU.
If you want to present e.g. a dual-core CPU to the VM, select 2 CPUs and set Cores-per-socket to 2.
Since you have selected CPU 1, which is absolutely 1core per socket. which will not give you another option.,
Here in you case as a.p. said since you selected 1CPU, you cannot increase cores, make 4CPU and play with cores per socket.
Another key factor to remember is the number of licensed CPUs on the host.