Well what kind of game is it first of all? What kind of enemies are you looking for? Where do you want them to appear? How often? etc.
You'll have to give more details before we can help you.
As Doggy said, there is a myriad of ways this can be achieved.
Also, welcome to the forums!
Chromegloss - Vanishes and then re-appears at complete random. [Working on Advanced Section of VBScript guide]