If only two centuries are destroyed, then the cohort would, considering that all centuries were at full-strength, still have 4 centuries. Now admitting that each of the surviving centuries lost ten men each, and that this cohort isn't the first cohort, the total remaining soldiers is of 280 men, a little above half-strength.

Now it all depends in the scenario.

If it was a cohort of the late republic fighting in gaul, they would be kept in the front, independently of the losses.

A cohort of the early empire in britannia, would be sent to the main base to wait for reinforcements. If its a cohort fighting against parthia under Trajan the same would apply.

Now a cohort fighting in germania, under Marcus Aurelius would probably be kept with the main army and would act as a reserve, to the main battle-line. That or be sent to garrison duties. Either way they would had to wait for the reinforcements.

Now in the late empire a half-strength cohort would be normal. As such they would be forced to fight, independently from the losses and would only be removed from the fight, if the cohort was on the brink of annihilation.

So resuming:

• They would had to wait for reinforcements and then wait for their turn to pick on extra soldiers. The first cohort would have first pick, followed by the VI, VIII and X, the III and V would come right next, then it was the turn of the II and IV and finally it came the turn to the VII and IX. This would change depending on the casualties each unit had.

• Auxiliaries to the legions would probably only apply after the edict of Caracalla. So only late roman legions would do that.

• If a cohort was lesser in number and it was no possibility of reinforcements to arrive early the cohort would be broken and its remaining effective's would be sent to other cohorts.