change amount of VPP apps

if you purchase 150 licenses for your existing app within a few seconds the total license will show as 200 in ABM
Jamf reflects this change shortly after that showing as a increase pool of available licenses.

There are no upgrades in a sense with VPP, the apps are continually updated by the manufacture
So as a example if you purchased 50 license of "Widget" version 1.0 in 2017 it could be "Widget" version 7.0 by 2020 - you still have 50 licenses of the current version version 7.0

its only different when the App developer changes the title of the App (which is effectively a completely different App entry in the AppStore) to force you to purchase again. Example is "Thingamajig 2" (version 12.5) which has been regularly updated for several years now has a very large update to "Thingamajig 3" (version 1.0) and the developer wants the App to be re-purchased to get all the new features

The amount I purchased in the past was 50 (version 5.13). I now only can purchase another 50 (version 5.15). So I purchased 2x50. In jamf this is visible as 50 (5.13), 50 (5.15) and 50 (5.15). In the self service app store there are now three identical
icons of this app. It's really confusing for my users. Besides, I still have only 50 licenses available.

Are they all on the same VPP token and are you sure you're set it up with VPP Device Assignments and not VPP Codes in Jamf?
It looks like you have 3 separate policies to divide licenses up

You are right. We recently migrated to Jamf, there was another instance in https://business.apple.com/. I have to migrate the licenses to the Jamf instance. Thanx!