when will apple stop upgrades for the iphone 6

They already did. The iPhone 6 cannot run iOS13. They may issue updates to fix bugs or security issues, but there will no more upgrades.

Upgrades have been already stopped last September, since that model can not support iOS version 13 or later.

Regards

Giulio

iOS 13 is already no longer supported. And no one knows if there will be any more iOS 12 Bugfix Updates.