According to a report, the Android Market now has 500,000 apps, but not all of them are active.

The Android Market has reached a milestone, hitting the 500,000 submitted apps, mobile research firm Research2Guidance said in a new report.

While it still lags behind the nearly 600,000 apps published in Apple’s App Store, the two figures are not exactly comparable. Google ascribes to an “anything goes” policy with the Android Market, letting in any app submitted. iOS apps, on the other hand, are subject to Apple’s strict approval process before they’re allowed into the App Store.

Research2Guidance also noted that there is a difference between Android developers and their iOS counterparts. Android devs are more prolific, with the average publisher releasing more than six apps to the Android Market. The average iOS developer has added around four apps to its respective store.

Research2Guidance predicted back in August that by the end of 2011, the Android Market would catch up to the App Store in terms of number of apps submitted, and Android is certainly closing the gap. But these numbers carry a caveat: just because an app goes live, doesn’t mean it’s around forever.

For example, by September, 37 percent of all Android apps and 24 percent of iOS apps had been removed for various reasons. An app could be killed if it’s not updated to be compatible with the platform’s current operating system, or if the developer abandons it, among other reasons.

This means that the actual number of active apps is 319,161 for the Android Market and 459,589 for the App Store. Apple is still ahead, but considering it debuted the iPhone about a year before Google went live with Android, the growth rate for the Android Market is undoubtedly impressive.

Reps from Google did not immediately respond to a request for comment on the report.