There's nothing in the text that says you can't attack it, so you're free to do so. As written, there's no advantage or disadvantage given to anyone involved when attacking an attached Wretched.
Do note that, while attached, you're free to attack the Wretched, but it can't attack you (or anyone else). But, it deals its damage automatically at the start of its turn without having to make an attack as long as it's attached.
If one of The Wretched bite you and attach on to you, do you use an action on your turn to remove it?
If you look at the stat block, it answers this question:
You can, but you don't have to. You're free to leave it attached if you'd rather use your Action for something else.
Okay that's what I thought, but can you attack it while it's on you?
There's nothing in the text that says you can't attack it, so you're free to do so. As written, there's no advantage or disadvantage given to anyone involved when attacking an attached Wretched.
Do note that, while attached, you're free to attack the Wretched, but it can't attack you (or anyone else). But, it deals its damage automatically at the start of its turn without having to make an attack as long as it's attached.
Thanks for the clarification
How many wretched is the regular number in the "pack"?