If you are wearing a shield you can block two monsters before it goes to armor. When wearing a shield it's the defense of the shield and any modifier on the weapon you are wearing that counts. The defense modifier on a weapon is the +x at the end of the stats.
If you are wearing a one-handed weapon or two-handed weapon you can block one monster before it goes to armor. When not using a shield it's the defense of the weapon without any modifier that counts.
AKA, the current "best answer" is wrong.
Source:
https://tibia.fandom.com/wiki/Shielding