I definitely buy more if something is on sale, especially if it's food, hygiene, or household products. In fact with most products, I will only buy them if they're on sale and, if possible, paired with a coupon for maximum savings. I typically get as much as I and/or my family and friends can reasonably use before the item expires, and I also buy a lot to donate to the homeless shelter and food bank. By buying items on sale (+coupon), I'm able to stretch my budget plus make substantial charitable donations, all while allowing me to focus my money on other areas that I otherwise wouldn't be able to fund if I didn't shop sales.
It depends on whether I need it or not. If it is something that won't go bad I tend to stock up. Sometimes I get things for others too. I like to have things on hand. Especially since I really don't like to go to the store. If the item was more than 50% off and something I use on a regular basis I would most likely buy several.