How to control Object Generation Speed.

I am completely to this platform.
But i am programmer.
But i am unable to understand how to control the object creation speed.

I tried to write the event to fire bullets from the canon but at a time four bullet where hit.

It happens because the condition to fire (for example, Left mouse button is pressed), is true for several frames. To limit the number of shots by time you can use a timer:
When the timer is greater than 0.3 seconds and the mouse button is pressed, shoot and reset the timer: As the timer is reset, the next frame the condition won’t be true, it can be true again only 0.3 seconds after shooting.