Not so fast there.
The requirement of 20 copies sounds like business, not personal use. The terms and conditions are different for businesses. You cannot buy one copy and have 20 employees (or 20 students if this is for a class being taught) using it on 20 different computers.
MAC APP STORE PRODUCT USAGE RULES
Except as otherwise set forth herein,
ℹ If you are an individual acting in your personal capacity, you may download and use an application from the Mac App Store (“Mac App Store Product”) for personal, non-commercial use on any Apple-branded products running Mac OS X (“Mac Computer”) that you own or control.
(ii) If you are a commercial enterprise or educational institution, you may download a Mac App Store Product for use by either (a) a single individual on each of the Mac Computer(s) used by that individual that you own or control or (b) multiple individuals on a single shared Mac Computer that you own or control. For example, a single employee may use a Mac App Store Product on both the employee’s desktop Mac Computer and laptop Mac Computer, or multiple students may serially use a Mac App Store Product on a single Mac Computer located at a resource center or library. For the sake of clarity, each Mac Computer used serially by multiple users requires a separate license.
(iii) Use may require sign-in with the Apple ID used to download the Mac App Store Product from the Mac App Store. Mac App Store Products can be updated through the Mac App Store only.