To create a Discord server folder you simply need to drag one server over another, this will automatically create a new folder that contains both of the servers you just dragged on top of eachother. You can continue just dragging servers into this folder as many times as you like, then when you are done you can right click on the folder to bring up a context menu with a few options. If you click on the folder settings option you can give the folder a name and background color, for example you can create a folder for fortnite servers and give it a purple color.
Discord server folders are a great organizational tool that lets you clean up your Discord server list. For example you can use server folders for keeping all of your gaming servers, emoji servers and friend servers separate. You can also mark entire folders of servers read at the same time which is extremely useful if you get a lot of pings in the servers you are in.
If you want to delete a Discord server folder, you can too. All you need to do is drag all of the servers out of the folder and it will automatically be deleted. This is really useful if you accidentally create folders a lot or you want to re-organize your server list.
Server folders are client based, which means they are not account-wide. You will need to create folders on every device you use manually.
You can put as many servers in a folder as you want, even all of them if you like.
Discord added server folders in summer of 2019.