As per Apple's licencing policy, for personal use, an iOS app (including Avaz) can be installed on all the devices that a user owns  (iPad, iPhone, etc.) If you own more than one iPad, you can sync all the devices to the app using the same iTunes account. For academic and commercial use, if there are multiple devices used by multiple users, you need to buy one license of an app per device. If a single user uses one or more devices, you need to buy one license per user.

For more information regarding the app licensing, please read the Terms and Conditions of Apple’s EULA (End User Licence Agreement) which is read and agreed to by the user, before purchasing the app. 

Click to read : Apple Media Services Terms and Conditions

The section APP STORE PRODUCT USAGE RULES states that

(i) If you are an individual acting in your personal capacity, you may download and sync an App Store Product for personal, noncommercial use on any iOS Device you own or control.

(ii) If you are a commercial enterprise or educational institution, you may download and sync an App Store Product for use by either (a) a single individual on one or more iOS Devices used by that individual that you own or control or (b) multiple individuals, on a single shared iOS Device you own or control. For example, a single employee may use an App Store Product on both the employee's iPhone and iPad, or multiple students may serially use an App Store Product on a single iPad located at a resource center or library. For the sake of clarity, each iOS Device used serially by multiple users requires a separate license.

(iii) You shall be able to store App Store Products from up to five different Accounts at a time on a compatible iOS Device.

(iv) You shall be able to manually sync App Store Products from at least one iTunes-authorized device to iOS Devices that have manual sync mode, provided that the App Store Product is associated with an Account on the primary iTunes-authorized device, where the primary iTunes-authorized device is the one that was first synced with the iOS Device or the one that you subsequently designate as primary using the iTunes application.