This should be an accurate list of enemy formations and where they are fought-- the absence of location information indicating an unused encounter. WM event encounters aren't noted though. I
believe for each of them the first listed is the used one (e.g. Ruby Weapon: 982 used; 983, 984, and 985 unused). For Ultimate Weapon they're all used except the one on the black background, and for Mystery Ninja 296, 297, and 298 aren't used but the rest are.
(Edit:
another explanation of WM encounters)
WM encounters are in enc_w.bin in the wm LGP. Blocks are 6 normal encounters, 2 back attack, 1 side attack, 1 both sides, 4 chocobo encounters. Each encounter is two bytes, First byte is encounter ID, second byte is encounter rate, although it only uses the first six bits (apparently the remaining two bits are also for the encounter ID, although all encounters used on the WM have an ID < 256). Each block starts with two bytes, first byte is 0x01 if encounters are set, 0x00 if not. Second byte is the area's enc rate, which does take into account all eight bits.
There are four blocks to an area of WM (the WM pole-placing place in Great Glacier is with the Icicle Area lot).
For example, this is the first four (starts at offset 0xA0) for Midgar
01 20 26 40 27 40 28 40 29 40 00 00 00 00 2a 08 \ Midgar - Grass
2a 08 00 00 2b 10 00 00 00 00 00 00 00 00 00 00 \-------
01 14 20 40 21 40 22 40 24 40 00 00 00 00 23 10 \ Midgar - Dirt
23 10 25 28 00 00 00 00 00 00 00 00 00 00 00 00 \-------
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 \ Midgar - Jungle/desert? unused
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 \--------
01 40 2c 58 2d 54 2e 54 00 00 00 00 00 00 00 00 \ Midgar - Beach
00 00 00 00 2f 20 00 00 00 00 00 00 00 00 00 00 \-------
Breakdown of Midgar Grass (EDIT: corrected after response)
01 -- Encounters on
20 -- Encounter value 20
26 40 -- 1st normal encounter 038, enc value 16
27 40 -- 2nd normal encounter 039, enc value 16
28 40 -- 3rd normal encounter 040, enc value 16
29 40 -- 4th normal encounter 041, enc value 16
00 00 -- 5th normal encounter N/A
00 00 -- 6th normal encounter N/A
2a 08 -- 1st back attack encounter 042, enc value 2
2a 08 -- 2nd back attack encounter 042, enc value 2
00 00 -- 1st side attack encounter N/A
2b 10 -- 1st both sides encounter 043, enc value 4
00 00 -- 1st chocobo encounter N/A
00 00 -- 2nd chocobo encounter N/A
00 00 -- 3rd chocobo encounter N/A
00 00 -- 4th chocobo encounter N/A
00 00 -- Unknown (always 0x0000)