What is better 1 bigger sub or 2 small ones

Will you be using the sub(s) as bass extension from 2 channel audio, or as a .1 dedicated sub channel?

The frequency response of the main speakers needs to be the first consideration for sub matching, whether for one or two. My Maggies start rolling off around 50 Hz, so my small, fast subs are a good match even though they only extend down to around 36 Hz.

However, the C-1s measure down into the 30s, so the sub(s) should be strong from around 35 Hz on down into the 20s to turn your rig into true full range.

We don't know your budget; you could possibly get a pair of JL E10s for around $3K and get meaningful bass down into the mid-20s, or you could get a $3-4K single JL that would take you down to 20 Hz flat.

The main thing is, for the C-1s, don't get subs so small they don't add meaningful bass extension and strength to what the C-1s can already do. From there, get two if your budget allows, or one if it doesn't.

I position my subs symmetrically alongside my Magneplanar 1.7s so there are no phase or imaging issues. You can also get good imaging if you center a single sub between the speakers so the mains and sub are all equidistant to the listening area.

Taters-

good to hear you selected REL subwoofer(s). These are the best in the market! The most musical, easiest to integrate in any set-up. Start w/ 1 sub for awhile. Get used to it and allow it attempt to transform your experience.

1 sub is certainly the easy set-up. 2 subs can present a more challenging aspect. Your room is going to play the major role here- is it "live" or not.
Keep me posted & happy listening!

I use 2 Rel T7s with my Cremona Ms with great results, highly recommend a dual sub set up.

Look at the specs of the different REL models and get the one with the lowest
possible frequency rating you can afford. My trusty old single REL Q150e
(10" mosfet little monster) is located directly behind one of my main
speakers, is all the sub my room can take, and it sounds great. 25 to 50hz is
where it lives (I've tested it with a tone CD...I recommend trying that as the
results are interesting) and the theory of "non-directional" low bass
is actual fact in all but the largest and deadest rooms...however, 2 subs will
make sure neither of them works very hard, it will put a smile on the salesman's
face, and will make you appear to be a very serious hifi owner to others. I
suggest you try one REL and after dialing it in simply see if it's doing its job to
your tastes. Instead of buying another one, spend the bucks on that REL wireless
gizmo....a seemingly great idea.