Silybum marianum is the botanical name of the plant.
Silymarin is the name of one active ingredient. It seems they have extracted that ingredient.
I would use the whole plant, not just one ingredient. (I know many people use silymarin with beneficial results.)
How much you take, depends on what you are trying to achieve.
Do you have liver disease? Or are you just taking it for prevention?
A suggestion, if you have liver disease, may be one gram twice a day. Less if just for prevention.
If you have liver disease, take it for months. But you should see an improvement in weeks. When you get your next liver tests see the results, and determine if you need to continue. People who have had liver disease, should take a maintenance dose for life.
Silybum marianum promotes regeneration of diseased liver tissue, and has saved the lives of people with liver disease.
The liver is one organ which can regenerate. People have had 90% of their liver surgically removed, and it has grown back again.
A commonly used pain-killing drug is causing serious liver problems in patients.
I have whole seeds, not the powder. You can chew them and eat them, but they are hard like raw wheat seeds, so many people may prefer to crush them first.