How can i define the stock value shown in the matrix?

You can define the stock value shown in the matrix ( ) in the catalogue from the stock settings.
1. Go to the stock module
2. Choose the correct order type tab depending on the item displayed, and mode of login.
3. Go to the 'displayed stock matrix' option and select your 'current stock'. This option only determines the displayed stock value in catalogue, so customer can still order backorder if this is set/available.

See also more information in article How do I determine the stock display in the catalogue?