The number shown in the upper left corner of the Mac App Store is the number of Extras application for which an update is available.
Will thus simply click on the Mac App Store, go to the Updates section and install the updates desired.
A graphic example of the icon of the Mac App Store for updates is as follows:

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CSS (Course)
Web Design and Accessibility according to W3C CSS and XHTML. Starting from 29 €. |
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E-commerce with ASP (Ebook)
ECommerce and Shopping Cart with ASP. Only 35 €. |
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OpenOffice (Ebook)
The open-source software for managing the office work. Just 25 €. |