Such monitors usually have a horizontal resolution of about 5 thousand pixels. Most likely, your 5K monitor will have the 5120×2880 resolution with a 16:9 aspect ratio, enabling it to display 14.7 million pixels.
Basically, it is 7 times better than Full HD with 1080p resolution. Such devices also have a 4 times higher linear resolution if one compares them to 720p screens.