SAAMI specs are I think 17,000 psi for 38 special and 35,000 psi for 357 magnum. So the pressure with 357 mag is higher by a factor of 2.059. I don't know about guns in particular but in general fatigue damage is proportional to stress to some power n. Now if n = 3, 2.059^3 = 8.7, so one might expect one round of 357 mag to do as much damage as about 9 rounds of 38 special. if n= 4, one round of 357 mag would be equivalent to about 18 rounds of 38 special. So the rule of which you heard makes sense in order to prolong the life of the gun. If you don't mind refurbishing or replacing the gun more frequently, then you can fire as many 357 magnums as you want.