Given that some variants reverse the colours of the pieces, or even use colours like silver and gold, I tend to use the term "attackers" for those attempting to capture the king, and "defenders" for those who try to stop aid his escape.

The reason I say twelve pieces is that the corner squares are often used as "hostile squares" for capturing, so eight pieces standing adjacent to them would be vulnerable for attacking. With twelve pieces, though, you could have a formation of pieces in squares A3, B2, C1 and the corresponding squares in the other corners. These would be invulnerable.