Seems like open is a requirement, since not all the numerals are closed and I'm not sure how one would morph between open and closed spines. Playing around here (http://www.victoriakirst.com/beziertool/ anyone know a similar tool that has bookmarkable results so we can share work?) I'm thinking three control points should be achievable (haven't found a way to make a nice looking 4 yet, but most other numerals aren't that hard to pull off).

Here is a quick upload of all the numbers (0-9) built with only 3 points. Sorry about not animating them... didn't have time. https://www.youtube.com/watch?v=yddpzyQoy_0&feature=youtu.be

Nice. Seems unlikely we could get it down to 2 points as numerals like 3 seem to require two distinct curves, but perhaps someone will try?