Solarpanel for charging iPhoneXs max

You would be better off  charging a battery / power pack in your rucksack as you walk about, and plug your phone in to the power pack. This way when the sun goes down you can still charge your phone / torch via the power pack.

I can guarantee your solar panel will not be the correct degrees to the sun as you will be mostly walking when the sun is up, and camping when it goes down?

So I would get the biggest flexible panel you can get that fits the back of your rucksack. Use a charger made for the job, as rapid charging will kill the life of the battery.