How do I know if my training data set is sufficient?
We can not judge whether our training data set is sufficient or not directly from the shape description for your data. 

At the most of the time, we sample some instances, hopefully they are distributed as real-world data, so that the final model constructed through these data can reveal the truth of the real-world. Theoretically, the size of training data is not the problem, but we shall collect training samples as many as possible to ensure our model will have good performance on predicted instances.

Anyway, we can get the answer by testing whether the distribution of our predicted value is consistent with that of truth.