You're going to need a 10,000 BTUs for a 20x20 room. Here's the math:
20 x 20 = 400 x 25 = 10,000
A 12,000 BTU mini split system will be able to handle a 400 square foot room. Once you go over a 500 square foot room, you're going to need to go up to a 15,000 BTU mini split system. A 12,000 BTU mini split can usually handle up to 12,500 BTUs.
We have a split heat pump sizing calculator you can use to get a quick size calculation by square feet.