What is the max characters for a license plate in the usa?

Some states it is 6, others 7 or 8. Each is different, some can use a single - or space as a character, others do not. You may find mixing numbers helpful to ensure your tag is uniquely yours. If someone previously thought of it, they probably already have that plate. That is one reason, you usually need to think of several possible combinations to use, in the order you want the DMV to try them.

7 in my state. The US is a big place with 50 states and some territories. Every on of them have different regulations denoting the amount of characters allowed.

In my US state it is 7.

What state do you live in?

That depends on the state. Lic. plates are handled by each state, not nationally by the federal government.