What size needle should I use with Sulky 12 wt Cotton?

Use at least a 14/90 needle with both the 12 wt. and the 30 wt. Sulky Cotton. Depending on the thickness and density of the fabric(s) you are stitching through, you may need to upsize your needle to a 16/100 for the 12 wt. Cotton. Some machines prefer a topstitch needle when using heavier threads, but generally speaking, any 14/90 will work fine.