E6000 glue is our go-to adhesive for gluing on endcaps. Be sure to let dry, undisturbed for 24 hours before wearing your jewelry.
Available in two sizes:
The .18 oz. tiny tube is the size I use most often. It's hard to measure just how far one tube of glue goes when it comes to gluing on endcaps, but I feel very confident that each tiny tube is enough for at least 10 projects.
The .5 oz small tube is 2.7x bigger than the tiny tubes, but still small enough that you can use it up in a reasonable amount of time. It's cheaper per ounce and good for when you're going to have a marathon gluing session.