I'm trying to research for a paper about why we shouldn't have flat tax rate, and a reason that i'm seeing people putting online, is that a flat tax rate of say, 18% would decrease tax revenue?

Can someone explain how?
I don't understand how taxes work.

Income taxes take a part of a person's income.

If you make $50,000 a year and pay 10% of it in taxes, you pay $5,000.

A flat tax would mean that rich people would pay the same percentage of their income as poor people.