They will spin the upper stage, and release them a few at a time, then spin it up faster, and release more. [Source][1].

>In order to save mass, each of the 60 satellites will not have its own release mechanism, such as a spring. Instead, Musk explained, the Falcon rocket's upper stage will begin a very slow rotation, and each of the satellites will be released in turn with a different amount of rotational inertia.
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>"It will almost seem like spreading a deck of cards on a table," Musk said. There may actually be some contact between the Starlink satellites, he added, but they are designed to handle it.

  [1]: https://arstechnica.com/science/2019/05/spacex-to-launch-60-internet-satellites-and-deploy-them-like-a-deck-of-cards/