Hi Urhan - sorry for the radio silence.
I should make the game report how many lost seedlings you found really, so you can judge which levels you need to attack to get the achievement.
Spoiler:
You can do that now though if you like - you have to enable debug mode (ctrl-d), then press F6 to show your stats - if you expand the window (drag the corner) to show all the levels, you can see which levels have seedlings left to find.
Level / number of hiddens
1 - 9
6 - 57
9 - 160
12 - 100
14 - 100
17 - 49
18 - 110
20 - 223
total = 808
Hope that helps.