What plants like Epsom salts?

I know Orchids DO! Google will help.

Tomatoes and peppers

Any plant in magnesium deficient soil. Soil tests from your local extension office should tell you (inexpensive). Since it's not 'chemical', adding too much shouldn't hurt - it's just a waste. I do know tomato plants love it. You could search for plants that love magnesium.

Any flowering plant loves epsom salts. Just make sure you're using the plain ones not the lavender or other fragrances...not sure how they would do. I've used them around my roses for years and have the nicest bushes with the most and biggest flowers. Just sprinkle 1-2 cups (depending on the size of your plant) around the base of the plant then let nature do it's thing and dissolve it. You can also add it to your watering can if that's easier for you.

Roses. Not sure of any others.

How much does one use?

Roses.

Two sites. This one gives excellent info about WHY Epsom salts are good for plants: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/epsom-salt-gardening.htm and this site with specific plants to use it on: https://www.seasalt.com/bath/gardening-with-epsom-salt