Ideally place in centre of area to be covered horizontal upside down attached to ceiling.
Obviously varies with specific needs and especially building construction materials, line of sight opportunities, interfering radio/electrical sources, accessibiltiy for installation and areas of higher client density.
This orientation works well for my 25 APs covering large school over two sites:
common scenario 2 APs support 6 classes arranged diagonally opposite ends of classrooms lying 3 on top of 3, in extreme case it is block of 4 on top of 4. Low client density, poss 60 clients over the 2 APs at max. Obviously they are not in centre ceiling of each class but against near walls to improve coverage into next room.