I cannot understand when it is necessary to use several nodes, as I understand on two nodes the system will be more secure. Should I use multiple node in a private network Stellar?
This is not a Stellar specific question: in a private network configuration, the core network is like any other service or backend.
Basically, to answer this question you have to go back to business needs. The more replicas, the more resilient the system is going to be across fault domains.
