Current ArcGIS Server Enterprise licenses (basic, standard, and advanced) are for 4-cores. There are "additional cores" licenses availalbe that give you "blocks" of 4 more cores.
However, in most cases a 8-core machine would be virtualized and you would actually run 2 to 4 VM's on the same hardware. In that case you would just assign the license to one of these VM's.
Enjoy,
Rich