This is life-safety, not a convenience tip. A jack can fail from a small bump or an odd angle in a second. One minute of setup could save your life.
For a quick roadside tire change on flat ground you may only use the jack briefly and never go under the car. For ANY work where your body goes under, a jackstand is mandatory — jacks fail.
Two is right, one per side. A single stand is a bare minimum; never rely on the jack alone.
More tips: · Make Creek or Rain Water Safe to Drink With No Power · Stop Bad Bleeding and Treat Shock · Running Your Car in Snow · Communications When the Grid Goes Down · Keep Your Phone Alive in the Cold · Your Freezer Is a Grid-Down Food Vault · Get Unstuck From Sand, Snow, or Mud · Stay Warm With No Power (Without Carbon Monoxide Killing You)
General preparedness information, not professional advice. For medical, structural, or dangerous situations, get real help when you can.