In macOS, a handy option called “Show suggested and recent applications in Dock” (found in System Settings ➝ Desktop & Dock) adds a divider to the right-hand side of your Mac’s Dock, and after it displays any apps you recently used – or any that macOS thinks may be useful to your workflow – that aren’t permanently docked.
Assuming that you have no undocked apps that are currently open, the right side of the divider shows three of the most recently used apps that have since been closed. However, there is a way to make it show more, which can be useful if your workflow involves using a lot of undocked apps consecutively.
If you’re comfortable pasting commands into Terminal, it’s perfectly possible to increase or decrease the number of recently opened apps that show in your Dock.
Read more at MacRumors.com