Here's how I would do it:
var possible_enemy_scenes = [load("res://Enemy_A.tscn"), load("res://Enemy_B.tscn"), load("res://Enemy_C.tscn")]
var number_of_enemies = 20
func _ready():
randomize()
for enemy in number_of_enemies:
var random_position = Vector2(rand_range(0, 720), rand_range(0, 720))
instance_enemy_at_position(random_position)
func instance_enemy_at_position(pos : Vector2):
var enemy_to_instance = choose_random_from_array(possible_enemy_scenes)
var enemy_scene = enemy_to_instance.instance()
enemy_scene.position = pos
add_child(enemy_scene)
func choose_random_from_array(arr: Array):
arr.shuffle()
return arr.front()