How do you know how much garlic to add to a dish and in what form?
Garlic is a wonderful vegetable that can add intense flavor, and is also praised by many people for its healthful benefits. But as is the case for most strongly flavored foods, personal preference varies widely.

You've already established one reference point, in that the amount you've used in the past was insufficient to result in a flavor that you personally found strong enough. So, the obvious next step is to increase that amount gradually each time you prepare that particular recipe until you reach a point you find most pleasing, or at least until the garlic flavor sounds a note that harmonizes with the dish.

Note that garlic cloves vary not merely in size, but also in intensity of flavor; depending on the source, age, and other factors. With experience, you'll gain a sense of this, partly from the aroma and "feel" of the garlic. As is the case with many cooking techniques, with practice you'll become better at judging this too.

Also to consider, is what form you're using the garlic in your recipe. For example, a meat roast recipe may call for whole cloves to be inserted, causing the flavor to disperse from the inside out. A sauté or sauce calling for minced garlic, distributes the flavor more evenly.

Using a garlic press goes even farther, releasing juice and making it almost like a paste — this works very good in a marinade, but it might be too strong for other things. You can also throw whole cloves of garlic in with potatoes while steaming them — the garlic will become buttery soft and blend in beautifully to make delicious garlic mashed potatoes.

If your garlic is burning, most likely the heat is too high, or else you're cooking it too long. At what point you add it to the dish, depends on what you're cooking, and what other ingredients are included. If I'm sautéing up a collections of vegetables, I'll usually begin with onions in the skillet first, and mince up the garlic while they're starting to cook. The onions will have "sweat" a bit, reducing the chance of the garlic burning.

Your task now is to experiment with all of these things, observing the differences in how it appeals to your own taste. If you only ever cook for yourself, you'll be all set. However, if you're cooking for others, you may wish to moderate the garlic level, to suit the tastes of your guests — for that, there's no rule of thumb.

If you're cooking for diners who normally eat bland or packaged food, a little garlic can go a long way. But for people who are used to ethic food where garlic is prominent, it might be impossible to use too much.

The key to seizing control of garlic and mastering it in your kitchen, is to explore and understand how it tastes to you. Doing so will give you the knowledge and skill to wield garlic with confidence. Have fun, enjoy garlic, and listen to feedback from your guests.