In the Rule book it states, "If a guard is the target of a melee attack, you must remove the guard marker when the attack is complete, at the end of the Counterstrike Step, even if the attack did not hit. Once the attack is resolved, the creature is no longer guarding and loses both the Counterstrike trait granted by guarding and the ability to protect its zone."
So even though it states that you remove THE guard marker, assuming only one, it also states that you won't benefit from guarding anymore. In that case if you are allowed to have more than one marker you at a time, you still won't have the associated traits after the first is removed.