What number do I put on the back of my son's new football shirt?

[Chucksey]

I'm buying my son a Chelsea football shirt for his birthday with his name and number on the back. He said he wanted the number 1 on there but that's the goalie's shirt which is a different colour. I can't question this with him as it's meant to be a surprise but I don't want the other kids telling him it's wrong and then he doesn't want to wear it. I've absolutely no knowledge of football so I'd appreciate some advice on my purchase.