I need advice

Okay, I want to start a Harry Potter roleplay in which Harry discovers he has a twin sister. The problem is that I am not sure where I should place this.
I am uncertain if I should make this a 1x1 or if it should be an advanced roleplay with many people, any advice for me?

That depends: do you want to be the twin sister? Because if so, you might want to have an environment where you find lots of different reactions to this, in which case I'd suggest several RPers. If you want someone else to be twin so you can show Harry any everyone reacts, go for a one on one.

I intended to be the twin sister, Zelda. Thank you for the advice.

I would suggest care with concepts such as this. The sacrosanct item known as 'canon' has not just a few fanatics guarding it zealously.