In a 55 gallon?? 12-20, depending on the amount of water changes you want to do. I am a fan of overstocking african tanks to disperse aggression. I would do 15 or so. They love caves and rocks structures.
First one is Melanochromis Auratus the second is metriaclima estherae (it USED to be p. zebra , P. Zebra has been divided into like fifty more species!). Both malawi rock dwellers, best to keep tons and tons of them together so the aggression is spread out more. Try the "African Rift Lake Cichlid" forum for more info.