The simple answer is **one extraction per portafilter filling.** 
So depending on whether you use the smaller one-shot or larger double-shot insert, you get one or two shots per button press *and after that, the grounds are considered spent*. 

If you’d try to leave the grounds and push the button again and again, you wouldn’t just get increasingly weaker brews, but you would notice a lot of unpleasant flavors, from excessive bitterness to sour notes. It may seem wasteful, but that’s how espresso works - you pretty much get the best of each load and that’s it.

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Anecdotally, I know of a somewhat strange habit in a few Swiss households that will keep once-used Nespresso capsules (which can count as “a serving of espresso grounds”) and will use it once again for a purposely weak brew that goes into making „Kafi Fertig“ („finished coffee“), a concoction that gets spiked with a generous amount of „Träsch“ (a fruit brandy) and sugar. But that’s not a practical option for a spent coffee pick which can and probably will fall apart, not to mention the food safety issues if you tried to keep it for a while. I very much don’t recommend it.