By reusing the stock solution. As you mentioned, Ilford recommends adding 10% development time after each film for ID-11. You get up to 10 rolls with this.
Kodak's D-76 instructions aren't quite the same. Their recommendation is adding 15% per roll, but only after the first four rolls are processed. This brings you to a total 16 rolls per litre.
Important to note that the used developer always has to be mixed with the unused part after each film developed.
Kodak D-76 (info on page 5 of the data sheet posted above by Luxaeterna) :
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Ilford ID-11 (page 8 of data sheet) :
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