If you use X-Tol, the developer is it's own replenisher.
You mix up 5 litres of developer from the package, and then divide it into at least two containers - a working solution container that is at least as large as your largest developing tank and then one or more containers for the rest - it will be used for replenishment.
Each time you develop film, while the film is being developed, you put 70 ml of the replenisher into your working solution container for each 80 square inches of film being developed (a standard 120 or 135-36 roll of film is equivalent to 80 square inches). Then when the development is finished, you pour the developer back into the working solution container. There will be some excess, which you discard.
The developer activity will vary slightly at first, but once it stabilizes, it gives results that many consider better than one-shot.
Over time, you may wish to experiment with slightly more or slightly less replenishment. And if you use unusually sized films (like 70 mm) you will need to calculate how many square inches of film are involved and adjust the replenishment amount accordingly.
Once you use up the balance of the 5 litre package, you just mix the next one, and use all of it for replenishing.
One 5 litre package will provide replenisher for about 70 rolls. Thus, this is best suited to those who shoot 70 or more rolls each 6 months.