the problem could be because you created them at the my.x position of the first box, try using a random value, like:

Code:

---

action enemy_creator
{
	var pos_;
////////set the maximum distance from the spawn box
	var max_x_ = 300;
	var max_y_ = 300;
	var max_z_ = 0;
	my.passable = on;
	my.invisible = on;
	while(1)
	{
		while(enemy_count < 5) //max_enemy_count = 5
		{
			vec_set (pos_.x, my.x);
			pos_.x += random (max_x_) - random (max_x_ *1.5);
			pos_.y += random (max_y_) - random (max_y_ *1.5);
			pos_.z += random (max_z_) - random (max_z_ *1.5);
			ent_create("speeder.mdl",pos_,enemy_flight);
			enemy_count += 1;
		}
		wait(5);
	}
}

---

i also changed the "if" to a "while" so that it should stop, although if it doesn't it'll create millions more, also, have you tried making max_enemy_count a local var instead?