For EDDM counts you can use the EDDM Every Door Direct Mail https://eddm.usps.com/eddm/customer/routeSearch.action to search for routes and get counts for zip codes, but we prefer a slightly different method. Doing large scale EDDM mailings we have found that the counts on the EDDM website are not always accurate.
Maintaining accurate counts is a difficult task for the USPS as homes and businesses are constantly being constructed, left vacant, and routes are constantly changing. A few things the EDDM counts do not take into account are vacancies or an address which has one receiving mail box but lots of addresses (like an assisted living facility). This can result in over printing and paying for additional printing and postage for pieces that will not get delivered.
At Action Mail we prefer to get counts from databases we have access to with full address information. These would be the same addresses we would print onto a piece if the mailing was not EDDM. These list are updated every month and remove vacancies and drops with an address where it has multiple households living at the address, but there are no room #s or apartment addresses. The counts also remove people on the Do Not Mail list.
These counts tend to be more accurate but the data still comes from the USPS and is not always accurate. The accuracy also depends on what the carrier does with the mailing and how the carrier reports vacancies.
We prefer using the more conservative counts as we do not want our customers paying extra for printing and postage for postcards or magazines that will not be delivered. Yet, we can use any counts as long as they are withing a certain range of those published by the USPS. At Action Mail we always prefer to address mail pieces as the likelihood of delivery is much higher.