DCE Sample Size calculation

I am about to use this formula for a Discrete Choice experiment with 4 attributes each contains 3 Level. Thus, I have some question that I am cordially requesting to get an answer for : - How to fixe the true proportion p as the the true choice proportion of the relevant population? - What relation can be provided between the DCE parameters (attributes, profiles ..) and the sample size calculation formula in term of fixing the minimum sample size required for the full study?