Login to Ad Manager. Scroll down to the bottom of the page and click on:

Click Zone Configuration on the next page:

Make sure you're in the right web property. If you're not, just click on the drop-down menu and select the property you wish to work on:

Once you have the right property selected, scroll down the list of zones to the ad position you wish to add an impression limit to and click the pencil icon that is to the left of that ad position:

On your right, go to the Limit field and type in the desired impression limit. Make sure to select the Year and Month that this limit is in effect for:

When you have finished setting up the impression limit for that Year, Moth and Position, click the save button.

If you need to set the impression limits for other ad positions or Months/Years, follow the steps above. You will need to click the save button before moving to a different position or month/year or your work will not be saved.