I thought you had to have 10 draws in a row, then a double ko before friendship showed up? (I was more into SF than MK )
anyways- with the guild boards and messaging feature, you can pretty much ask someone to put up a key card you need for a specific board so i see no reason to limit the friends to 20. also, at any given time- a majority of my friends end up running a holy priestess so I'd rather have a bigger pool to pull from.