Create a new object or add to one of your "general" objects ie. "obj_game" which governors the game.
There you make a simple counter...
Code:
If alarm[0] == -1
{
alarm[0] = 30
global.counter += 1
}
Then the spawning code of you're enemy embeed in if global.counter > 9
Of course course alarm must be set to you frame rate and global.counter must be given in create event with 0 value.
If you don't need the counter later on in the game you can add one more if global.counter < 10