You need at least 4 to reach the heart at an upward angle under the ribs/sternum. 5 or 6 if you want a more effective stab. Slices can be devastating but it's easy to sew up even a sliced open belly. They won't live long enough to get help with a hole in the heart, an effectively severed renal artery from a deep puncture in the lower back or heavily lacerated liver.
If it comes to the point where I need a weapon to make you stop whatever you're doing, I don't want any story but my version being told.
Slashes are easier to deflect/ reduce the impact of with blocking. Grappling with someone and pulling them into a braced blade in your hand is a lot easier than finding an opening while extending your arm and slashing, and also a lot easier to retain control of your weapon. Any time your arm extends towards your opponent you're giving them an opportunity to take control of your weapon hand. If you're going to use a knife in an active fight, keep it close to your body and pull them into you. Leave as little space between your body and your knife hand or theirs and the blade as possible. Use their own weight on the blade for penetration. Get the knife inside their torso and then slice and move through until you find something important, and don't let go of them or pull the knife back out until resistance stops.
If they also have a knife, gain control of it first. Grab the wrist, twist outward and use that as leverage to pull them in to you, extending your arm as far out and behind you as possible to keep their blade away from your body and force their body into your blade.
I wouldn't say that. And I don't mean to come off as a know it all. But when choosing a weapon to defend my life with, "should work" isn't good enough for me. I define objectives of purpose, and pick the design that is intended to meet those objectives.
Your every day pocket knife might do the job in a pinch. But if you are picking a knife to carry with the purpose of winning a knife fight, you need to pick a knife that was designed for that purpose and understand the objectives you want to achieve in that fight.
Control of your opponent's weapon, retaining control of your own, and inflicting fight stopping/ deadly injury as quickly and efficiently as possible.