On the yolo website, there are weights pretrained on the coco dataset, which consits of 80 objects/classes, (and I think also weight for voc pascal dataset which consist of 20 classes). Are there (out there) some weights that have been trained on a dataset with far more classes? I was thinking YOLO can detect thousand of objects and not just 80 as it seem to be the case. Or I'm missing something?
It's not likely that you'll be able to find weights trained on more or less than 80 classes thought it's reasonably possible. You can recover the full repo pjreddie/darknet from GitHub and then change the config file in order to train your model on how-much-you-want classes.
The full description awaits in the repo's README.md file.