Yes Piranhas can eat humans but it would take more than one Piranha to actually eat a whole entire human.
But even one Piranha can eat the flesh off a humans hand in just a a minute or less so never stick your hand in a Piranha fish tank or anywhere where Piranha fish are present.
Piranhas really don't make good pet fish either so I would not recommend the Piranha as a pet fish because they're dangerous fish and really should become extinct.