The amperage rating of the breaker is the number that is stamped on the breaker. If it says "15", then it's a 15 amp breaker. If it says "30", then it's a 30 amp breaker. In other words, you are not required to do any math to determine the amp rating for the breaker.Since it is a double pole breaker, it is double the Voltage of a single pole breaker installed in the same panel. So, if your panel is 120/240 volts like a standard residential panel, then your double pole breaker is rated for 240 volts.
Here's the reason I asked: I have an old oven that I think draws 40 amps but it's fed by two linked single pole breakers marked 20 amps. Can that be? Should the individual breakers both be marked at 40 amps?