Adding a sub. - can i start with one?

I’m pretty happy with my setup I’ve assembled over the past year. However I feel I am missing some oomph and I find I am turning up the volume a lot. My space is big (converted barn) with 18 ft vaulted ceilings and about 700 sq ft of space with hardwood floors and minimal soft furniture. 
 

So I am thinking adding a sub woofer will help increase the bass and maybe give me more of the kick in the sound I’m used to in my old room and prior older speakers and amp. 
 

I’ve read that most people add 2 subs to a system instead of one. I’d prefer to spend the 2500 per side over time and just get one for now. What’s the thinking on one channel sub vs two and can I consider going from 1 to 2 later on as another upgrade?