Tower Defence style spawning(enemies)

basically what i need is enemies to spawn after i click start, but i want a cycle of enemies to spawn just like in plants vs zombies where it has a status bar saying how long left until enemies run out.

This is a big question i know but thats just saying what im hoping for. right now i wish to spawn say 5 enemies from a pool then when they die bring them back again. this cycle should last until say 25 enemies in total are killed.

I know im asking a lot but could any one lead me in the right direction or give me some simple code where i can spawn a couple of enemies? thanks a lot :D

What about the Unity tutorials? There are principles in there that would help you accomplish what you need... for example, Lerpz's enemy robots.