CompuServe Messages

I was attempting to render my tree stuff last night, and got the following error message: "Can't allocate object instance!" There are presently a little over 6000 instances of one object in the scene. Have I hit some kind of limit? I don't know where in the rendering process this error occured, since I wasn't the first one in this morning, and the person who wrote down the message didn't know to mark down what spot it died at. He just said he tried to keep hitting the continue button, and the message just kept popping up, like it was running through all the instances in the scene. Please tell me how to avoid this message. I have 20,000 more trees to add to the scene this afternoon. Thanks.