A hit is a request that you send to the service. Depending on the information you request, your response can result in 25 EANs/ISBNs or it can result in a single EAN/ISBN. It just just depends on the data you are seeking.
If you are trying to receive full data on an item, each EAN request will result in that EAN's information. If you are submitting key words to obtain a list, you will receive up to 25 EANs per hit, depending upon your request parameters.