I have a recipe that calls for 2 cloves of garlic, but all I have is minced garlic. How much minced garlic do I use per clove?
As even with garlic directly from the bulb, you need to taste, as @ElendilTheTall says. Adding is easy...
All other things being equal (clove size), garlic can easily range in tastes from stronger to weaker. And, the tastes of your target audience will be different as well.
Its no different, and arguably more so, with chopped garlic, presumably in a glass container. You also have the luxury of easily adding a little bit, tasting, adding more if necessary. Then again, getting your own from your local farmer's market (or growing!), smash/chop, is not really very difficult for many.
It hasn't been established whether you have dry minced garlic, which is basically just dehydrated garlic, or the wet kind, which is minced in a liquid. I've used them both, but prefer the latter. To me, it acts and tastes more like fresh.
Since there's some curiousity expressed in the comments, I'll answer this based on the assumption that you're asking about the minced in a jar with liquid. I apologize if it's the other.
Where I live in Massachusetts, in the northeast region of the United States, jars of minced garlic in liquid are available in almost every major grocery store. They're generally found in the produce section, but not always. I use it because I don't cook very often and fresh things tend to go bad in my house. It's also perfect for times like you're experiencing, when I want to make something and have run out of fresh garlic. I usually buy this brand, because they offer an organic version, and I use organic products whenever I can. (I just noticed on that site that they sell a version in a squeeze bottle. It looks interesting!)
The jar says 1/2 teaspoon equals approximately 1 clove of garlic, so if you want to stick directly to your recipe, I'd go with that, meaning 1 teaspoon for your total recipe. I agree with the heaping teaspoon advice you've already received, as I find it easy to underestimate. In my experience, it tastes very much like fresh, so that substitution should give you the same level of garlic flavor. However, if you use it in other recipes, or just want to experiment, there's nothing wrong with adding more. As you've been told, you can add ingredients but can't subtract them, so be careful. I advise tasting the product before adding it, to give you a sense of what to expect.
The way you measure it can be important too. If you're doing an exact substitution, it's best to spoon out mostly just the garlic pieces, being careful not to use much of the liquid in the jar. However, the garlic imparts flavor into the water and citric acid in which it's generally packed, so if I have a recipe where the liquid content is adjustable, I always add some of the yummy "garlic juice." An example of that is mashed potatoes.
Minced garlic works quite well as a substitute in many recipes, so I hope you'll enjoy the recipe that prompted this question!