How do I instantiate only ONCE?

Hi, I've looked everywhere for 'instantiate only once' and nothing has worked for me. Why does spawning in update, even with a bool, spawn clones on clones on clones?

What I need:

To create a ripple effect- spawn two or three circles in a row. Once the first prefab reaches a scale bigger than the screen, instantiate another of the same prefab.

So far I've tried - if (instantiated == false, instantiate, instantiate = true) and a if (prefab != null) ....

On that note, how can I make an object spawn spurts of a prefab every few seconds?

Thanks so much!