Look ok at the chart above from Poseidonrc's site. They have a rpm limit of 30,000 on their big 22 series motors. Now have a look at the Kv for each motor and the max voltage. 400kv multiplied by 75v equals 30,000 rpm, 600kv multiplied by 50v equals 30,000rpm. So if you go by their chart and get a 600kv motor there's no way it will exceed 30,000rpm on 12s unless it has no load on it and the batteries are fully charged. Put it in the vehicle, apply full throttle and the battery pack voltage is likely to drop to at least 4.0v per cell or 48v for the whole pack, 48v multiplied by 600kv is 28,800rpm.
Now that 28,800rpm is the max theoretical rpm the motor will reach but in reality it's going to be lower. First of all Kv is rated without a load. The exact Kv with a load depends on the amount of load and the motor's power output. A 600kv motor on 12s the Kv under the load of you're vehicle is probably going to be more like 500-550kv. So at 48v the max motor rpm is going to be more like 24,000-27,000rpm. This is a nice safety margin to ensure the motor won't reach its max rated rpm.
The max rpm rating of a motor is just the max rpm. It doesn't matter if the motor is loaded or unloaded, if it exceeds the max rpm problems can occur.