Windows 8 has just surpassed the 20,000 mark for the number of apps on its app store, with the majority of them available for free.

The store only opened in October after the release of Microsoft's latest operating system, with the company promising fast growth of the number of apps available.

The Web site Win App Update, which keeps track of the totals, said that the milestone was likely reached three days ago.

"Today's totals: 20,610 apps worldwide in the Windows Store (broke 20,000 on Tuesday, I believe). 17,958 of those are free (87%)," said the Tweet.

Not all the apps are available in every country; however, Win App Update says almost 13,000 are available in the US, 14,000 in Canada, and 11,000 in the UK.

At the start of the month, the Windows 8 store reportedly only had 12,895 apps. This means some 7,715 apps were added in the last 17 days at an average rate of more than 453 apps per day. That puts the company on course to hit 30,000 apps by the end of the year. However, the total is still a long way from the 100,000 Microsoft promises for its Windows smartphones, tablets, and desktop PCs available by Jan. 2013.

The biggest challenge for Windows, which is seeking to make a mark in the tablet and smartphone market, is competing with the vast number of apps on the Apple iOS and Android ecosystems. Both those platforms boast more than 700,000 apps each.

Microsoft clearly has a long way to go to reach those figures, and as recently as Tuesday Andrew Webber, who is the UK marketing lead for indie game app builders at Microsoft, said the company only had 1,421 game app.

Webber said that he expected this to change over the next year, but added that there was a great opportunity for developers to take advantage of the digital store while numbers were low, giving them discoverability that they would not have on Apple's App Store or Google Play Store.

The apps for Windows 8 are designed to work seamlessly between all its devices, including those running Windows Phone 8, such as the Nokia Lumia 920 smartphone or Microsoft's Surface tablet.