receptor reserve
Receptor reserve means a tissue has more receptors than it strictly needs — spare capacity sitting in reserve. A maximal response can be reached while many receptors remain empty, like a stadium that fills to capacity for the show even though it has extra seats.
This arises when the signalling machinery downstream of the receptor amplifies the message strongly, so activating only a fraction of receptors is enough to saturate the response. A practical consequence is that the EC50 for the effect can be much lower than the Kd for binding, because you reach half-maximal effect before half the receptors are occupied.
Receptor reserve has clinical bite. It explains why some agonists are so potent, and it determines how an irreversible antagonist behaves: at first such an antagonist only shifts the agonist curve to the right by silently using up the spare receptors, and only after the reserve is exhausted does it start to lower the maximal response. Reserve can also shrink in disease or with receptor down-regulation, changing how a patient responds to a drug.
In some smooth muscle, a full agonist reaches its maximal contraction while occupying only about 1% of receptors, leaving a large reserve of spare receptors.
Spare receptors beyond what is needed.
Receptor reserve is a property of a particular agonist in a particular tissue, not a fixed feature of the receptor; a low-efficacy agonist may show no reserve in the same tissue where a strong one shows plenty.