# How to select the order of LPC to model formants?

I am having some difficulties understanding LPC and speech processing in general. The professor asked us to provide an heuristic way to select the order of LPC? For example if I wanted to model 3 formants what would be the order I should use? I understand the order of prediction is the number of coefficient used in the estimation of the signal $$s(n)$$: $$\hat{s}(n)=\sum_{k=0}^p a_ks(n-k)$$

I am also open to any resources for a beginner in automatic speech recognition.