It will depend on the variety of heuchera. They don't all grow to the same ultimate size. And you will have to decide how quickly you want coverage, versus how healthy you want your plants, and how often you'll have to divide/renew them. Ideally, your plants will be allowed to reach their individual potential. No plant does well when really crowded.

For general gardening purposes (not instant coverage!) choose the variety you want (I'm assuming here that it will be just one variety. Things get complicated otherwise). Research it's ultimate size. Divide the total square footage you want to cover (in your case, it's 32 square feet) by the square footage of a single plant. Most heuchera will cover at LEAST one square foot. Some, much more. If your chosen type says it will grow to 12", that's approximately 32 plants you would need so that each plant grows to touch its neighbor. If the plants are 18" across, you would need 21 or 22 to fill the space. Add a few more to make sure you don't see the ground. Larger, more robust growers will take up more room, so you'll have to figure accordingly.

If you're buying MATURE potted plants, which you probably intend to do since you want instant coverage, you'll have to know their size NOW, and use that to calculate the square footage of each plant, then divide 32 by that number. But again, it will depend on the size of EACH plant.

Thanks very much!!!! Great info!