It's been a little while since Windows 10 for phones technical preview was released for users enrolled in the Windows Insiders program. Unfortunately, the preview is not available for flagship devices like the Lumia 1520 at the moment, with the most high-end device in the compatibility list being the Lumia 830.
According to a report from Neowin, who cite an unnamed source, there are only 60,000 users testing Windows 10 for phones among the Windows Insiders -- the number apparently includes Microsoft employees as well so the number of normal users should be a bit smaller. But as mentioned earlier, flagship devices are not yet supported so it doesn't come as a surprise that the user base testing the new operating system is so small. There are also roughly 2.8 million Windows Insiders.
We're hoping Microsoft will introduce an update for the preview and introduce support for flagship devices as well. We've done a quick preview of the features introduced with Windows 10 for phones, take a look at it to see if there are any surprises.