Adding replinisher to a developer will extend its capacity. But not indefinitely. In the case of D76 used with replenishment, Kodak recommends dumping the developer after 9600 square inches of film per gallon have been processed.
Some replenishers require prior removal of an equal amount of old developer before adding to the tank. Others are sufficiently concentrated and so little is required that they simply replace the developer which was carried out on wet film in normal processing. Manufacturer's instruction sheets will give directions to follow.
Some developers are replenished with a special replenisher which contains concentrated chemicals which are depleted in development. Other developers are replenished with new developer instead.
The amount of replenisher to use is determined by the amount of silver which is developed. This, in turn, is calculated by the number of films developed, the exposure and the tonality of the subject matter. Obviously, high-density negatives exhaust the developer more quickly than thin negatives. Calculating replenishment solely on the basis of films run will inevitably lead to big errors over time, making the developer much to active or much to weak.
For this reason, labs which replenish also develop pre-exposed test strips of film from the manufacturer, which are read using a densitometer to check the developer activity level. The amount of replenisher added is determined by these density readings. A lot of extra work over simply diluting developer and using in the "one shot" method.
Unfortunately, some machines and some developers require replenishment.
Here is technical information on replenishing D76 from Kodak:
http://www.kodak.com/global/en/prof...78/j78.jhtml?id=0.1.16.14.32.14.7.14.14&lc=en