We cannot give a definitive answer for the maximum pressure for ferrules.
Ferrules are intended to hold tubing in place. The maximum pressure for holding the tubing is dependent on different factors. Such as the type of tubing; a hard tubing will hold better with a ferrule than soft tubing which can and will eventually slip out of the ferrule hold with high pressure.
And of course, the force you use to make the connection makes a big difference. The harder you put the fitting onto the ferrule the more pressure the tubing / ferrule connection will hold. However, be careful not to apply too much pressure to avoid deforming the ferrule.
It is difficult to state the maximum pressure rating for ferrules for these reasons.