What is the maximum niceness value that a regular user can assign to a process with the nice command when executing a new process?

9 is a valid nice value a regular user can set, but it is not the maximum - users can go up to 19.

Nice values range from -20 (highest CPU priority) to 19 (lowest CPU priority). Regular users are restricted by the kernel to only increase the nice value (reduce priority), and they can set it as high as 19. Only the root user can assign negative nice values to increase a process's scheduling priority above the default of 0.

49 exceeds the valid nice range entirely; the scale only goes up to 19.

99 exceeds the valid nice range entirely; the scale only goes up to 19.