Adding a sub. - can i start with one?

a single sub works best, if you have a single listening position, and the flexibility to place the sub where it sonically needs to be...the more subs, the easier they are to place...

Probably the real question comes down to getting the ideal sub for your room  (the perfect choice of type and brand) and one that gives the sound and flexibility you want. I went with dual Rythmik subs because they are quick, easy to set up and dial in for two channel listening

I only have ever used one sub, when I even have used a sub.  It's been fine. 

But then I don't have a 700 ft w 18 foot cathedral ceiling room.  However, if your main speakers are big enough to handle such a room, then probably one good sub would do it, especially if you're already "almost" satisfied except for a bit of oomph.

OP, it seems that there are some answering the question whether one sub will be "enough" and others are answering what will be "ideal." Which are you after?

I’ve heard Rel subs and they work great, but so do many other subwoofers. There’s nothing special or unique about Rel subs that I have heard. I’m sure fanboys will say they weren’t set up "right" or I’m deaf, or whatever, but that’s my story and I’m sticking to it.

I’ve nothing against other sub manufacturers such as SVS, Rythmik etc. I’ve been using a sub for the past 15 years or so and never really got a satisfactory result blending it to the main system. It went in and out of the system(s) throughout the years, used in Home Theater for few years and was kept in storage unused for several years.

Just lately I bit the bullet and switched to a REL S/510, and it’s one of the best decisions I’ve made although initially I was rather hesitant to consider another sub upgrade after all the years of disappointment. I’m glad I did and better late than never. The integration between the sub and main speakers is seamless which is the most important aspect in a music only system. Nothing sticks out like a sore thumb as the overall sound quality has noticeably improved, not night and day but surely appreciable. And this is only with one sub.

Coming back to the thread, to reiterate all that’s said here the consensus is one is generally considered to be better than none, and two is better than one. Or course, three or four is even better but one has to draw the line somewhere. It’s all about managing expectations. For me, one currently works very well in my system, and since I don’t have the space or funds for a second one, I’m already done.