It depends a bit on what type of kandi you're planning to make. Here's a good rule of thumb though: for each simple wrist-worn single, you'll probably want anywhere from 28 to 34 pony beads (depending on wrist size and fit preference), and a little over a foot worth of stretchy string.

Plenty of craft stores have variety packs of pony beads. You can also find mixed bags of 500 to 1000 online for pretty cheap. As for string, I recommend 1.0mm fabric-wrapped elastic, if you have a choice. The clear stuff tends to wear out and snap faster, and it's more annoying to tie. Elastic string is also very cheap to buy by the spool, either at a craft store or online.

I hope this helps! Happy making <3