“There’s a twenty dollar minimum.”
You’re a bartender. A customer tries to pay with a credit card. Your bar gets busy, so you don’t allow customers to use a credit card for just one drink. You say:
There’s a twenty dollar minimum.
The word "minimum" means "the least amount". You can use it like this:
You know, a lot of people just try to do the minimum amount of work that they can get away with.
The minimum age is 18.
Some stores have a credit card minimum. This is the smallest amount of money that you can use a credit card to pay for. For example, if you try to buy something that costs $5 but the store has a "$10 minimum", they will ask you to pay with cash instead.
Stores, businesses, schools, homeowner associations, and other groups have rules. One way to talk about those rules is to say "There's a ___." For example:
There's a strict dress code.
There's a limit of one item per customer.
There's a no-smoking policy.