How to decide filter order in Linear Prediction Coefficients ( LPC ) while calculating formant frequency features?
You have to define the order based on your application, usually for speech you'll use and order of 10, this definws the order of your all pole filter and it's related with the amount of formants tha you will see in the impulse response of the filter. 

https://www.dsprelated.com/showthread/speechcoding/965-1.php