1 Large vs Many Small
Many small is better so your fish will not get diseases, plus one large system if it fails your pretty much in deep water.
Above ground vs In ground
In my mind it's better to start above around first with smaller breading tanks. When you have more fish build a pond and grow them out side for them.
Is shape important?
Kind of, I like raceway systems myself or tanks you can stack on top of each other to save space so you can have more fish, but in reality as long as you can circulate the water and you have a great filter it doesn't really matter.
Is Depth Important?
Yes, if your going to put out side then you don't want your system to freeze solid. You also want to things, lots of water as a buffer so you don't have to do constant water changes and you also want a lot of surface area so the toxic gas and will escape your pond or your tank. Plus more surface area means more surface aeration. If you ask me the more surface area is better then having a deeper pond or tank.