Ducati used to name their model lines from engine displacement. For example Supersport 750, 900; Superbikes 851, 888, 916, 996, 998, etc. However, those were nominal figures - the 900 bikes were actually 904CC and eventually 944CC, etc.
As engine modifications for torque -vs- displacement changed they stopped numbering them this way and moved instead to the xx# format - the xx indicated "displacement" and the # indicated the model line. Example - 749/999, 848/1098, 939/1299. And then there's the 859/1099/1199 Panigale engine line.
The most recent model line on the "small" Testrestretta engines is the 939 and used in the Monster, Hyper and Supersport. The "large" Testestretta engines are 1299 and found on the Multi and Monster, and in a couple of years, probably on the SS.
The take away is that the number is indicative of an engine displacement "range" and model line, which is why I found it funny when someone here "added" the new Supersport to that sprocket calculator tool as a "937" since no such model of Ducat actually exists.