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I would like to use a YOLO network, retrain it to recognize say, 20 labels.

Would adding labels to a single network in training reduce its accurary?

Would it be better to train multiple models on 5,10 and 5 labels respectively.

e.g.

Model A : recognizes 5 labels (ferrari,honda,ford,toyota,tesla)
Model B : regonzies 10 labels (store front / stopsign, speed sign, super charger station , etccc. )
Model C : recongizes 5 labels (pedestrian, dog, marathon runner, bicycler, scooter rider)

OR

Model D: Recognizes 20 labels (all labels above put together)

Would Model D be as accurate as Model A,B,C put together?

It depends specially on data size and model capacity.

1. If the number of data points per category is insufficient, which is frequently the case in reality, a single model for combined categories performs better, since the effect of data shortage will be attenuated by sharing a model among different but related tasks, i.e. shared knowledge. However, we should not forget about the capacity of model, if model capacity is not enough for the combined task, this sharing would backfire as the model spreads its limited capacity across multiple tasks and becomes "jack of all trades, and master of none". Therefore, this approach is favorable if model capacity permits, which is less of a concern for neural networks with flexible size constraints.




2. If the number of data points per category is sufficient, which is rarely the case, or model capacity is limited (for example, when model performance stops improving when trained on more than 10% of data), single model per category is better since no capacity will be wasted on different (although related) tasks. From a different point of view, although this choice might be less favorable in terms of model accuracy, but it allows parallel development on smaller tasks, i.e. faster production, which could be a justified benefit for the cost we pay in terms of model accuracy.

We can also combine the two approaches, which could be more time-consuming than both. First we train a model on combined categories, then we feed a part (or parts) of the trained model (such as a layer, the final prediction, etc.) to a specific model that will be trained on one category. This is exactly what a pre-trained model does for us, it is trained on a more general task with more data (possibly even a different but related task), and then we use it for a specific task.