I think training from scratch on 50 pictures has a great probability to overfit on training data, thus a pre-training on a larger datasets (MS-COCO) and transfer-learning techniques could be considered.
Besides, you could adopt some data augmentation techniques such like cropping objects and paste them to compose novel training samples, which can enrich the training data at some extent.
Moreover, if you can have access to additional unlabelled data, then some other works like weakly-supervised object detection or semi-supervised object detection could also be considered.
I'm going to train a object detection model, But my training set only has 50 pictures,Can you give me some good advice,