can I trade in more than one iphone

You should be able to trade in as many devices as you own. Go to this link and scroll down to find the FAQs - http://www.apple.com/iphone/trade-up/

You do not have to buy two iPhones, you will be able to trade one device in directly towards a new iPhone and they will provide Apple Store Gift cards with the amount they agree to give you after examining the devices.

If you trade-in two iPhones you may have to buy two new iPhones. See the pic below. It gives you option for only one iPhone trade-in to buy one iPhone

https://www.apple.com/shop/buy-iphone/iphone-xs