yes, the wrong amperage can burn up a relay, but the question here is..... If you know it is a 12v automotive relay, then why would you need to know the amperage? say the real good alternator puts out about 30 amps, but the circuitry in the auto doesn't allow it to get to the relay. Then your relay might only require a few amps to fire.
But there are very few auto manufacturers who would put a small relay in because it is possible for power surges, or mechanical errors that could allow high amps through.... so they usually use high amp relays(anyway they used to.... before you kids got their hands on a VOM and a DIP package) Only way to know, the specs of anything, is to ask the manufacturer.