I did it once and used 900 bodies. I believe that there is no specific number for this task. I will leave some comments that may help in something else.
According to tibiawiki:
"Tibia wiki note: Corpses should be thrown into the lake until the contamination is precisely reached, there are no official numbers, but he believes that it varies between 800 and 1000 bodies and some say that bodies that have no volume (Cobra and Butterfly for example) do not count for the advancement of world change. Note that this number is cumulative, so if you play 100 bodies a day you will reach your goal in just over a week. To check the state of the lake, just tell the fish to Yawno and when the goal is reached, he will say he needs it and asks you to come back the next day. After the server saves the lake will be brown and extremely dirty."
Twisted Waters - Tibia Wiki
And according to tibia fandom: "The exact number of corpses is 1000, accumulated on the server."
Twisted Waters World Change | TibiaWiki | Fandom
You can also guide yourself through the progress of global change through the Guide NPCs, are the following messages:
the lake is clean: The big lake near Port Hope is clean.
If the lake is about to get dirty: corpses are piling up in the big lake near Port Hope and the water is about to get dirty.
If the lake is dirty and fishy: The big lake near Port Hope is dirty. Bright swimmers can be seen under the surface.
If the lake is about to get clean: The big lake near Port Hope is dirty. No brilliant swimmers have been seen under the surface for some time.
Just say hi - twisted waters to any guide and they will answer.
Remember, you can fish every 20 hours only 1 fish per character good luck.