Add timer behaviour to player object.
Have a global variable for shuriken ammo that starts at 5 and reduces as you use them. For shuriken reload you can say if ammo less than 5, start "timer" for 3 seconds, on "timer" completed add 1 to shuriken ammo. So if shurikens are not full it will keep adding more ammo until max is reached.
For shuriken fire have a global bool variable that is set to true by default, which means you can fire a shuriken. You can say on shuriken fired (button pressed), start "timer" for 2 seconds, and set bool to false. Then say on "timer" completed, set bool to true. This locks out firing of shuriken for 2 seconds.