The simple answer is that the number of sprints before a release is defined in the Sprint Review. The Scrum Guide identifies the following as a suitable item for Review:
"Review of the timeline, budget, potential capabilities, and marketplace for the next anticipated release of the product"
Please note : In Scrum, each Sprint must result in an increment of functionality that is potentially releasable at the discretion of the Product Owner. There may or may not be a cadence to those releases, and so the number of sprints before an actual release may or may not be constant.
There are other frameworks (such as SAFe) that seek to establish a release cadence, but that isn't part of the Scrum philosophy.