I believe you can't place any more than the limit in the standard World Editor. Not too sure though, does it allow you to bypass the limit?
I've been creating a huge Warcraft 3 map for almost a month and now I discovered that I can't add more trees, the following message "Maximum Number of Trees Exceeded" After that, the map editor simply didn't save anymore, so I had to delete the trees. I've already tried using another object and...
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A nice solution and one that would also help with map performance is to rely on custom tree models, especially batches of trees that all exist in one single model file. Remember, a Destructable is more complex than a Doodad because it can be destroyed. So if your Destructable is purely visual (unable to be interacted with or destroyed) it would make more sense to use a Doodad. Unfortunately, it's not so simple with Trees since they rely on special Destructable fields to handle their textures, that's why I suggested a custom model.