A reason your offer is limited in quantity. The sibling of urgency.
Where urgency is about time ('before Friday'), scarcity is about quantity ('only 20 spots'). Both create a reason to decide now. Scarcity works because of loss aversion, humans hate missing out more than they love gaining. Genuine scarcity (capped cohorts, limited inventory, one-time releases) is powerful. Fake scarcity destroys trust.
Scarcity creates decision pressure. Use only when real, and make the math obvious. 'Only X left of Y' is how you communicate scarcity without sounding like a used-car salesman.