When I've joined TibiaQA, I kept this question saved so I could answer it based on my own experience, since I wanted both achievements and also Groam bosstiary from the Awash World Change.
I've activated the world change two times until now, and I can safely say that the number of Deepling Scouts that needs to be killed everyday to keep the world change active is either random, or is irrelevant.
In the first 3 days, I killed more than 100 Deepling Scouts thinking that the number could be around 100 or 200, but I've noticed that after killing 128 Deepling Scouts in the first day, the second day I killed more than 200 and the Guide NPC wasn't updating the change status, I felt that it was like a progressive amount of kills to keep it 5 days fully active, something like:
Day 1 -> 100 Kills
Day 2 -> 200 Kills
Day 3 -> 300 Kills and so on..
But it doesn't work like that, because on the 3rd, 4th and 5th days I've killed less than 200 Deepling Scouts and the world change status still progressed.
So the conclusion that I've came up with is that the amount of Deeplings that needs to be killed is irrelevant or either random.
It's like the world change will keep active all 5 days unless no Deeplings are killed. Maybe all other Deeplings counts too, because the last days I killed less than 100 Deeplings and before the server save I made sure to check and found out that even with only a few kills it would still progress. I'm still testing and maybe will try to kill the least Deeplings possible, but I still need to kill some to pass through the 1 SQM corridors to check Groam, so it's hard to not kill them as they block the path, but these are my numbers until now:
1st Awash World Change numbers:
1st day = 138 Deepling Scouts killed
2nd day = 250 Deepling Scouts killed
3rd day = 177 Deepling Scouts killed
4th day = 82 Deepling Scouts killed
5th day = 125 Deepling Scouts killed
2nd Awash World Change numbers:
1st day = 83 Deepling Scouts killed
2nd day (today) = 61 Deepling Scouts killed
As you can see, it is not a fixed number, it is either random or irrelevant, because on the second day when I killed 250 Deeplings, even after 200 kills it was not progressing and only progressed after I killed 250, but I felt like that was not necessary to kill 250 everyday since as the day passes by, the progress is natural and automatic, even if you only kill a few Deeplings like the days I killed less than 100.
And then after 5 days, the mines flood again no matter how many Deeplings you kill, then you need 202 Coal to drain the water again.
As for the boss Groam, I'm on my second Awash world change and found him only in the first day of the second World Change, didn't found him in any of the 5 days of the first one, so I concluded that he works just like the other Nemesis bosses that may appear at any time of the day and also despawn shortly after appearing. I got the Eye of the Deep achievement and it was right after server save that opened the mines on Day 1 of the second Awash world change.
I wish you good luck!