You can find the number of cores your computer or server has by going to Start, and typing in msinfo32 to activity the system information utility. In the System Summary scroll down to the entry for the Processor and this should indicate the number of cores.
Keep in mind that the number of Agents installed on a Relativity server can't exceed the number of cores. Relativity agents work in the background to complete user initiated jobs. For example, it's possible to add more worker OCR agents to complete an OCR job - just don't exceed the number of server cores!