Here is my suggestion how to make perfect Transfer System:
As you will have fulll Servers and you don’t want people to transfer aimlessly filling up servers again so fast, that nobody can join and other servers beeing left emtpy here is what to do.
Make a Transfer Ticket System.
You have now full servers. So give people the choice to say, that they are willing to transfer.
This server has now it’s virtual population reduced by 1 so full server 2000 goes down 1 player but only in the transfer board, so other players, that want to play on this server can now apply for this spot. The virtual slot that is free is marked yellow as long, as this player has not changed servers. As soon, as this player changed to the other server, the spot becomes green. So for example, you got this number to be shown 1997 2 1
the 2 is yellow as 2 player want to change and the 1 at the end is green, as one player already left the server. You might also leave the full number and only show the number for the transfer that might happen and the ones that are really available.
The player that wants to change has to do two things for his/her transfer to happen. At first, apply for a server. Maybe also for a full server, but then the “Transfer” button (second step) keeps beeing grayed out until a spot becomes available.
And please use a applied queue. Who ever applied gets into a queue, so that player can transfer to a server with their friends, as soon as everyone of them has a safe spot to be transfered.
Also please consider giving players the opportunity to change to empty servers as having only full servers might end up in a situation like we are in now. So that players can’t play with their friends because they couldn’t transfer to a server together 
(to less empty spots)