My inbox shows -1/5 or 4/6 but there are only 2 messages in there.

Some database inaccuracy has developed within the MailCOPA email program. This is possible when there has been a system crash, or access to the database over a network has been interrupted.

The fix is to select Reindex from the Utilities menu.

MailCOPA Email Program Utilities Menu

Then select Foldes in the upper box, MessageList in the lower box and press the Reindex button. Observe the cautions issued!.

MailCOPA Email Program Reindex Form

Once this has finished, select Recount Unseen Messages from the Utilities menu.