Add deluxe backers to your pins order for extra security. They have a flat top that locks the backer onto the pin post. In order to remove the backer, you need to lift the top and pull off, taking two actions to remove.
Determining how many you need is dependent on the number of posts on your pin. Pins 1.4" or larger will have two posts by default unless requested otherwise.
100 quantity of a 1.5" pin would have 200 posts and need 200 deluxe backers
100 quantity of a 1.25" pin would have 100 posts and only require 100 deluxe backers
If you just want to add some to have as an upsell item, you can purchase any amount you'd like. Some people will offer deluxe backers as a $1-2 upgrade on their site.