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.
