The quota is in terms of elements:
Users of the standard API:
2,500 free elements per day, calculated as the sum of client-side and server-side queries.
Each query sent to the Google Maps Distance Matrix API is limited by the number of allowed elements, where the number of origins times the number of destinations defines the number of elements.
Adding more origins/destinations to a request won't increase the amount of distances that can be retrieved before hitting the quota.