Due to a code limitation, adding more than 50 API into an Application will output an error and the Application fails to be created.
There is a known limitation/defect (DE462630) in recent Portal versions (4.3.x, 4.4 and 4.5) where adding more than 50 APIs into an Application will thrown HTTP Error 500.
Release : 4.3, 4.4, 4.5
Component : API PORTAL
A permanent fix will be included in future Portal release.
Workaround: if you are in need to have more than 50 APIs into an Application, please add those APIs into an API Group and use the API Group when create the Application rather than single APIs.