One of the trial sized kits of Rebound was all I needed to get a mold from a full life cast (bust) of a persons head, so you might get by with just one kit. However, if the helmet sculpt is much larger than that I'd recommend two kits. Better to have too much than not enough, although if you run out you can just get more- silicon sticks to itself.
Additionally I would suggest you purchase Smooth-On's Thivex II additive. Just a few drops in your second and third coats will really improve the build of the material (no dripping 'cicles) as well as speed up the cure slightly. I've found it indispensable. The first coat goes on really thin to capture detail and the subsequent, thickened coats build up the rubber faster. Omit the thickener for your last coat so you get a nice smooth rubber surface.
Be sure to read Smoot-On's literature and check out their on-line tutorials, there's a lot of good info out there.
One more thing, and this is critical- if you use clay containing sulfur the sculpture absolutely must be sealed or the silicone will not cure and you will have an expensive mess on your hands. Rebound, being a platinum cure silicone is especially sensitive to this. A few thorough coats of spray acrylic sealer should do it. Make sure you coverage is total.