If i can have more then one Pierce rank, for example Pierce 1 from the weapon and Pierce 2 from a skill, I have a total of Pierce 3 or i have to take the bigger rank of Pierce, and then only 2 ?

Its in the rulebook, page 13.

Pierce from multiple sources are added together, which means that an attack with both "Pierce 1" and "Pierce 2" ignores up to 3 shields.