Hi @shinseung.
There is no parameter called --p-sampling-depth in alpha-rarefacion. Do you mean --p-min-depth or are you looking at the feature-table rarefy command that does have this?
If the former, this value is totally up to you and it only refers to the starting point of creating the rarefaction curves. I think 800 is totally fine there. If you are rarefying and are looking for a sampling depth, that is a whole other story. I tried answering this a bit here if you want to read further.