I've had a few questions on "how many beads do I need to crochet that pattern in 8's, 11's, 15's, etc." If you have an Android phone or tablet, you can download an app that will figure all of this for you. Here's some information on it...
Bead Crochet Designer's Bead Needs. I helped develop it, so I know it works quite well.
If you prefer to use pencil & paper to figure the beads needed, click on the link under the photo. The chart is limited to only 6-around ropes, but covers 5 different bead sizes. If you're out 'n about, buy beads, want to start stringing and crocheting right away, this is a quick and dirty calculation;
length of desired rope (inches) X number-around. OK, you want a 9" bracelet, in 8-around... simply multiply 9 x 8 for 72" of strung beads (estimated).
The Bead Crochet Ropes book does have a whole page of this kind of information and covers other number-arounds. You can find the
eBook edition over at www.bead-patterns.com .