So I was bored while waiting in queues and then I asked myself how many turtles actually made into water xD. Some raw data needed is presented below.
WoW EU Char amount : 1172984
WoW US Char amount : 1039382
WoW Rest Char amount : 62772 (TW) + 60109 (KR) = 122881
WoW Total Char amount = 2 335 247
Avg rep per quest : ~200
Avg daily number of WQs : 4
Avg daily number of saving the turtles WQs : 2
Number of turtles saved per WQ : 12
Rep gained from normal quests : 75+10+150+75+250+150+75+150+75+75+75+75+75+10+75+10+150+75+75+75+75+150+75+75+75+75+1000+75+75+75+10 = 3540
Rep needed to get to exalted : 3000 (neutral) + 6000 (friendly) + 12000 (honored) + 21000 (revered) = 42000 - 3540 = 38460
Avg WQs to get to exalted : 38460 / 800 = 48,075 day sets of doing all four WQs
Avg numbers of 'saving the turtles' WQs to exalted : 48,075 x 2 = 96,15

Assumption#1 : Reputation bonuses are disabled = true (not all play human or have always DMF up)
Assumption#2 : People will stop doing WQs after exalted = true
Assumption#3 : People start doing WQs with no rep what so ever = true (to simplify the math) = true
Assumption#4 : Even with lowered numbers data corruption is always present so let's say it's 10% of chances to get things wrong
Assumption#5 : RU realms counted as EU = true
Assumption#6 : Oceanic realms counted as US = true
Assumption#7 : Rest counted as TW, KR realms counted as one = true
Assumption#8 : Players' amount data (wowprogress, TUj) doesn't reflect the numbers of actually 120s = true (so it's needed to make assumption on how many 120s are there)
Assumption#9 : Based on heroic pugs / guilds I'm currently was/in I assume numbers of 120s @ 30-40% on at least 1 toon
Assumption#10 : First numbers of 'saved turtles' were too big so I had to manually count the rep given by normal quests yet side specific quest (ally or horde) are not counted and the source of rep amount is wowhead
Assumption#11 : For the sake of turtles (xD) I've rounded numbers to integers
Assumption#12 : For sake of dispute on numbers assume that a million is 10^7 (1 000 000) and a billion is 10^9 (1 000 000 000) due the mismatch between languages

Those numbers & assumptions point to this :
WoW Total Char amount x Avg numbers of 'saving the turtles' WQs to exalted x Number of turtles saved per WQ x Assumption#4 x Assumption#9

Variants
2 335 247 x 96,15 x 0,9 x 12 x 0,3 to 2 335 247 x 96,15 x 0,9 x 12 x 0,3

That leads to :
#1 808 mills
#2 835 mills
#3 862 mills
#4 889 mills
#5 916 mills
#6 943 mills
#7 969 mills
#8 996 mills
#9 1023 bills
#10 1050 bills
#11 1077 bills
...of turtles made into the water

Or raw data :
#1 808 322 397
#2 835 266 476
#3 862 210 556
#4 889 154 636
#5 916 098 716
#6 943 042 796
#7 969 986 876
#8 996 930 956
#9 1023 875 036
#10 1050 819 116
#11 1077 763 195
...of turtles made into the water

Now what to do with that kind of knowledge You may ask ? Well there was a topic https://www.mmo-champion.com/threads/1903704-Legion-Map-Comparison that provides (without BFA content) : 146 721 358 (146,721 mills) pixels. Now there is also https://www.youtube.com/watch?v=AyDiMPZxupM which provides : 14,01 mills pixels.
Total amount of all land mass pixels : 160,731 mills
Now if his math is not wrong then : 1px = 1,902m
160 731 000 / 1,902 = 84 506 309 meters
Now if I remember correctly pixel is not one dimensional thing - it's pixel per pixel size or in another words a two dimensional thing. That's something what we can work with. To sum this up we've got so far land mass of all Azeroth in number of ~84 506 km^2. So finally we can use snails' numbers in a manner like this :
Number of snails / pixel
Number of snails / meter^2
We've got this method numbers posted below (for simplicity & better reading - "S" stands for "Saved") :
Variant -- S.turtles -- S.Turtles [units not mills] / pixel -- S.turtles / meter^2
#1 808 322 397 5 10
#2 835 266 476 5 10
#3 862 210 556 5 10
#4 889 154 636 6 11
#5 916 098 716 6 11
#6 943 042 796 6 11
#7 969 986 876 6 11
#8 996 930 956 6 12
#9 1 023 875 036 6 12
#10 1 050 819 116 7 12
#11 1 077 763 195 7 13

More readable version of last one : https://imgur.com/7OZHdwy

Now conclusion....
1. Number of turtles per every square meter of land mass varies from 10 to 13.
2. Amount of saved turtles will raise if emissary system will be introduced.
3. Even with turtles' life is most in the water (90:10) we should see from 1 to 1,3 turtle / m^2.
4. Considering any resemble of the IRL turtles' life span we should be "drowning" in turtles any time soon.
5. Your effort to save the turtles in pointless cause they live their life in water (all of them) or they invisible (so there is no way to tell if you saved at least one) or they just simply die somewhere.