This is the number of products found in the domain. If a product is mapped to more than one domain, it is counted separately for each domain. (For example, if the same product name appears in both Children's Books and Fiction Books, the count for both of these domains will include that product.) This means you cannot sum the product counts to determine the total number of matching products across all domains. The histogram is only intended to show the number of matching products in each individual domain.
A product can be mapped to more than one domain. This means that even if a name appears in this histogram and the Count is greater than 1, you won't necessarily see the same name returned for each returned product (in the Product node of the response). That is, Product.DomainName only returns the most applicable domain name (as determined by eBay).