This chart shows which warp coil to use for some common bead sizes:
Delicas:
8/0- 9 per inch: Use the 8 dent spring
10/0- 14 per inch: Use the 12 dent spring
11/0- 19 per inch: Use the 18 dent spring except when doing very wide pieces, when you should use the 16 dent spring.
15/0- 25 per inch: Use the 22 dent coil just in order to space the beads.
Seed Beads:
15/0- 24 per inch: Use the 22 dent spring.
11/0- 14 to 15 per inch (sizes vary slightly depending on finish and manufacturer): Use the 14 dent spring.
8/0- 12 per inch: Use the 10 or 12 dent spring depending on what size warp you are using. For example, when using the bead cord, because it is thicker, you will use the 10 dent spring. But if you are just weaving straight beads using beading thread as warp, you will use the 12 dent spring.
6/0- 8 per inch: Use the 12 dent spring warped every-other-dent for 6 dents per inch.