Torches create light of 14 in the block they are in, all neighboring blocks will have light of 13 and so on.
Enemy mobs spawn at light level 7 or less
Placing one in each corner and two in the center covers all the places.
XX XX XX XX XX 08 XX XX XX XX XX XX XX XX XX XX 08 XX XX XX XX XX
XX XX XX XX 08 09 08 XX XX XX XX XX XX XX XX 08 09 08 XX XX XX XX
XX XX XX 08 09 10 09 08 XX XX XX XX XX XX 08 09 10 09 08 XX XX XX
XX XX 08 09 10 11 10 09 08 XX XX XX XX 08 09 10 11 10 09 08 XX XX
XX 08 09 10 11 12 11 10 09 08 XX XX 08 09 10 11 12 11 10 09 08 XX
08 09 10 11 12[13]12 11 10 09 08 08 09 10 11 12[13]12 11 10 09 08
XX 08 09 10 11 12 11 10 09 08 08 09 08 09 10 11 12 11 10 09 08 XX
XX XX 08 09 10 11 10 09 08 08 09 10 09 08 09 10 11 10 09 08 XX XX
XX XX XX 08 09 10 09 08 08 09 10 11 10 09 08 09 10 09 08 XX XX XX
XX XX XX XX 08 09 08 08 09 10 11 12 11 10 09 08 09 08 XX XX XX XX
XX XX XX XX XX 08 08 09 10 11 12[13]12 11 10 09 08 XX XX XX XX XX
XX XX XX XX XX 08 09 10 11 12[13]12 11 10 09 08 08 XX XX XX XX XX
XX XX XX XX 08 09 08 09 10 11 12 11 10 09 08 08 09 08 XX XX XX XX
XX XX XX 08 09 10 09 08 09 10 11 10 09 08 08 09 10 09 08 XX XX XX
XX XX 08 09 10 11 10 09 08 09 10 09 08 08 09 10 11 10 09 08 XX XX
XX 08 09 10 11 12 11 10 09 08 09 08 08 09 10 11 12 11 10 09 08 XX
08 09 10 11 12[13]12 11 10 09 08 08 09 10 11 12[13]12 11 10 09 08
XX 08 09 10 11 12 11 10 09 08 XX XX 08 09 10 11 12 11 10 09 08 XX
XX XX 08 09 10 11 10 09 08 XX XX XX XX 08 09 10 11 10 09 08 XX XX
XX XX XX 08 09 10 09 08 XX XX XX XX XX XX 08 09 10 09 08 XX XX XX
XX XX XX XX 08 09 08 XX XX XX XX XX XX XX XX 08 09 08 XX XX XX XX
XX XX XX XX XX 08 XX XX XX XX XX XX XX XX XX XX 08 XX XX XX XX XX
[13] = Torch
XX = Spawning available
Numbers show the lowest light level in that area
This can only work in a three high or shorter room with all torches on the middle row