One way to do this would be to use the dyn/admin to query the number of uses for the given voucher code:
- Open the ClaimableRepository in dyn/admin http://your-host-name:your-port/dyn/admin/nucleus/atg/commerce/claimable/ClaimableRepository/
In the text area below Run XML Operation Tags on the Repository enter the following:
<print-item item-descriptor="PromotionClaimable" id="my-promotion-id" />
and hit the 'enter' button. This will print something like this:
------ Printing item with id: my-promotion-id
<add-item item-descriptor="DeployablePromotionClaimable" id="my-promotion-id">
<set-property name="expirationDate"><![CDATA[11/26/2015 23:59:00]]></set-property>
<set-property name="displayName"><![CDATA[10% discount on my items]></set-property>
<!-- rdonly derived <set-property name="derivedDisplayName"><![CDATA[10% discount on my items]]></set-property> -->
<set-property name="uses"><![CDATA[4]]></set-property>
<!-- export is false <set-property name="version"><![CDATA[13]]></set-property> -->
<set-property name="lastModified"><![CDATA[10/28/2015 16:18:09]]></set-property>
<set-property name="status"><![CDATA[claimed]]></set-property>
<set-property name="startDate"><![CDATA[8/13/2015 00:00:00]]></set-property>
<!-- rdonly derived <set-property name="hasPromotions"><![CDATA[true]]></set-property> -->
<set-property name="promotions"><![CDATA[promo100001]]></set-property>
<set-property name="type"><![CDATA[DeployablePromotionClaimable]]></set-property>
</add-item>
The line <set-property name="uses"><![CDATA[4]]></set-property>
tells you how many times that voucher/coupon has been used, which in this case is 4.
UPDATE:
The above method does not work as it returns the total number of coupons available when the promotion is setup. This means the quickest way to get this information would be to query the completed orders which use the coupon.