Most .40 size planes use size 64 rubber bands, I think the SNS does as well.

You'll want to put on at least 8, and maybe as many as 12 rubber bands total.

The SNS instructions should show something about installing some sort of rubber band retainer. Usually this is just a dowel drilled though the fuse just in font of the wing LE and another one just behind the wing TE. Some SPADs use a bit of music wire bent in to a U around the bottom of the fuse, with the ends bent in to hooks for the rubber bands.

When putting on the rubber bands, it's a good idea to cross the last pair, ie, run them from front-right to back-left and vice-versa. It helps keep the bands from popping off in the air.