[...]Can we/ how many set subs like RMT/RGT can we bring and if so do we still need to provide GUIDs?
There is no set limit when it comes to substitutes. When you sign-up your roster, from our perspective, everyone is a player from your regiment and as soon as he/she played one official match under your banner, they count for being a full-part of your regiment for the duration of the RGL. We do not ask you to outline which players from your roster are going to be part of the main line-up and which ones will be the substitutes.
Concretely, this means that if you wish to sign-up your whole roster for this, and just not your competitive line-up, you can. In a sense, you are even encouraged to do so as we want the most matches possible to be bigger than the regular 15v15. Obviously, if big regiments sign-up with their full 80+ roster, we'll have a chat with their leader to make sure they understand that 1) There is no point signing-up players who will not partake in the event 2) Most regiments won't be able to meet your numbers anyways so you will end-up benching quite a lot of people each match. But at the end of the day, it's up to you to every leader to work out their own roster like they see fit. Previous iterations of the RGL didn't have a roster limit afaik and I don't think we'll be putting one for this edition either.