More than 50 android devices on same username/password
Google Play won't let you do this. You can only have a few devices attached to each account at a time. This is to stop people abusing the facility of using paid content from any device on their account.

If the app is an open-source one, or it's under your control, you should distribute it to all your devices through a different mechanism (e.g. a custom app list on F-droid). Otherwise, you should contact the developer of the app and ask to make a volume licensing deal. Unless they're already dealing with enterprise customers, they won't be prepared for this situation, but I think most app developers will be willing to help you out for a reasonable bulk price.