Over two months ago at CES 2017, Dell announced the UltraSharp 32 8K Monitor, a 31.5-inch display with a 7680x4320 resolution, giving it a pixel density of 280ppi. For those that are bad at math, that's over 33 million pixels.
Naturally, that's more pixels than any commercially-available monitor previously, and most current cables can't support it, such as Thunderbolt 3 or HDMI 2.0. DisplayPort 1.4 supports 8K at 60Hz, and you'll be using two of those cables for the full experience (note that at CES, we were told that it's powered by two DisplayPort 1.3 cables).
If you're interested in such a monitor, it's available for pre-order from Dell now for $4,999. It will begin shipping on April 14, which should be shortly after the Windows 10 Creators Update is released.
It's important to note that at this time, the UltraSharp 32 8K Monitor is not for everyone. Dell promised that the prices of 8K monitors will come down over time, as they did with 4K displays. Also, very few of the applications that most people use are built to scale to 8K, which you can probably see from the tiny taskbar in the image above.
This monitor was designed for those that will use it for drawing, graphic design, or other areas where the extra screen resolution will be truly useful. If you want something that will give you really clear text when you're reading emails, it's probably best to stick with 4K for now.