# flatMap(maxPublishers:_:) Transforms all elements from an upstream publisher into a new publisher up to a maximum number of publishers you specify. ``` func flatMap

(maxPublishers: Subscribers.Demand = .unlimited, _ transform: @escaping (Self.Output) -> P) -> Publishers.FlatMap> where P : Publisher ``` ## Parameters `maxPublishers` Specifies the maximum number of concurrent publisher subscriptions, or [`unlimited`](/documentation/Combine/Subscribers/Demand/unlimited) if unspecified. `transform` A closure that takes an element as a parameter and returns a publisher that produces elements of that type. ## Return Value A publisher that transforms elements from an upstream publisher into a publisher of that element’s type. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)