You can buy the extra coolant and take back what you don't open. When it's flowing out the top air bleed screw opening when the engine is warm then it isn't going to take anymore no matter what the book says.
There are a couple of good Youtube videos on how to do this. It is a messy job. I had no issues except trying to remove that stupid bleed screw on the top front of the engine. 8MM if I remember correctly.

 

I have a 2015 Rzr 1K. I just changed my coolant and the manual says 4.9 qts. 11+ qts. seems like a lot. Your 6 qts. seems more normal. I would buy 2 gallons and do your coolant change.I would bet you'll have plenty even after you bleed the air out. Changing coolant was a royal PITA, but it gets done. The biggest pain is getting to the air bleed screw up at the top of the front head

 

is is my concern, this is from the manual

Coolant Capacity

XP: 10.8 qts. (10.22 L) XP 4: 11.7 qts. (11.07 L)

mine is the 4
i did bleed it a couple time where the fluid came out