In the book it says " Weapons inflict critical wounds with a Sigmar’s Comet result on an attack "

However, I can't find anywhere that explains how many Critical wounds are inflicted… or are all critical wounds?

Count up the amount of Critical Damage that gets inflicted by an attack (max one for Comet, max one for spending boons equal to weapon CR, plus any additional from action card effects) and then flip over a number of wound cards equal to the number of Critical Damage inflicted.

Note 1: you can only flip over wounds that were inflicted in the current action - if you have more critical wounds that normal wounds, the excess criticals are ignored (but every wound you did inflict is a crit!)

Note 2: if the damage from the attack was insufficient to penetrate the target's soak value you will still do 1 damage, but with no chance of inflicting critical wounds. In this case, your count of critical damage will become the minimum amount of normal damage that gets inflicted. (This only happens if damage < soak and critical damage > 1)