While 20,000 apps in an online store may seem paltry compared to the more than 700,000 apps in Apple's store and more than 600,000 apps in Google Play, for an upstart in the app trade like Microsoft, the milestone is significant.

Microsoft broke the 20,000 mark on Tuesday, according to Directions on Microsoft Windows app store watcher Wes Miller. Nearly 18,000, or 87 percent, of those apps are free, according to Miller.

Those are worldwide numbers. The total number of apps available within regions vary. For example, the Canadian Windows 8 app store has some 14,000 programs, while the U.S. has 12,675; and the U.K., some 11,000.

What must be heartening for Microsoft is the velocity at which apps are being added to its online market: some 500 new apps appear each day, according to The Next Web.

At that rate, the store could reach 40,000 apps by the end of the year. Moreover, that climb should remain steep as buyers of new Windows PCs during the holiday season hunt for apps for their new computers for weeks to come, and developers feverishly seek to meet that demand.