What are the mandatory components of SOA ? how many of them need to be implemented to call the infrastructure as SOA based design?

SOA components depends on the design pattern you are following but there are some witch can bring the value in SOA like EMS, Service Inventory (UDDI), Web services.

The SOA components are: XML, Web Services, SOAP, WSDL, Discovery and the base components for a standard infrastructure are Web Services and contracts