You can do this with a registry change,
Whenever making registry changes be sure to do a full backup first, I am not responsible for your mistakes.
Open your registry editor (regedit)
right click on advanced and add new DWORD value "JumpListItems_Maximum" and assign the desired value in decimal format.
In most cases you do not need to restart anything, if you would like you can restart explorer using the task manager.
open task manager: Ctrl+Alt+DEL select task manager and under the processes tab right click on "Windows Explorer" and choose restart.
That's it, your done.