1 and a 1/2 1 and a 1/2

Depends on how many switches each router takes. You need a router to connect all the other routers, and then enough switches to make 81. For example, you can use 6 16 port routers to connect the computers and 1 8 port router to connect those routers together

To stack routers means to physically stack routers. Some stack a couple of routers on top of each other if they plan to just use them as switches or similar.

1

Wireless N routers have better range and much better transfer speeds than G routers.