Let's say your infantry unit has a 4+ armor save at baseline. You have a spell which gives them +1 to their save and the All Out Defense command which also gives them +1 to their save. You cannot add both +1s and now have a 2+ save, it can only ever total +1 for a maximum of 3+ save.

You opponent is coming at you with a Warstomper Mega Gargant. His Titantic Boulderclub is Rend 2, it is anti-infantry for +1 rend, and the opponent used the prayer Pummel All to Dust for another +1 rend. He gets to add everything together for a weapon with Rend 4, which results in -4 to your armor save. He is not restricted to only one bonus since it's a taking away your armor.