What's The 80% Rule?
The 80% rule is a requirement in the National Electric Code (NEC) mandating that any circuit that is used for a continuous load should be de-rated to 80% of its breaker rating. A charging EV falls into that continuous use category, so you should only charge an EV at 80% of the breaker rating.
In practice, if you use consumer EV charging products, they will obey this rule. So an EVSE that plugs into a normal 15A household circuit will only draw 12A. The biggest receptacle in common use is a 50A NEMA 14-50 or NEMA 6-50, and EVSEs that can plug into those receptacles will limit their charging to 40A.
Incidentally, there is a quirk in the electric code that allows you to install a 50A rated receptacle with only a 40A breaker. That's the reason why the Tesla Mobile Connector, for example, only charges at 32A with a 50A rated adapter plug. Since many consumers won't realize they need to check their breaker for a random 50A receptacle, the Tesla Mobile Connector errs on the side of caution and maxes out at 32A (of course, this makes the product cheaper, lighter and smaller too).