I have an iPhone 7 how much is it worth

If you want to trade it in with Apple, see this article:--> Apple Trade In - Apple

or

you can try and sell it on you own, at the best price you can (do some research on the Internet).

Regards

Giulio

Have a look here https://www.apple.com/shop/trade-in if you want to know how much you would get on a trade in.