How much media do you need? Seramis vs Perlite

[Nesuchambers]

Hi, I wanted to ask you guys about two things (I tried to reflect that in the tittle).

First, I have never repoted and I want to do a mass repot of all my orchids, how much media do you need on average? Im lookin up media in stores but I dont know how much will I need.

And second, perlite vs seramis? both hold a lot of water and give air to the roots, do have experience using them? wich one would you use?