When you are buying deployment licenses they come in 25 license packs. To buy deployment licenses please contact your distributor.
Also remember that if user in your application sign in to ArcGIS Online or Portal, it unlocks the standard functionality and no deployment license is needed for deployment. You can also save the authentication token to the application and that gives user access to standard functionality offline for 30 days (re-authentication needed before it ends).
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Also keep in mind that each deployment is a single use license - per app, per device. Concurrent use licenses are not available and there is no distinction between an internal (eg enterprise) or external (eg public) app.