My girlfriend is flying from China to US. I booked her ticket through cheap tickets and the passport number was not requested. Does this need to be on the ticket, or is just her name on the ticket sufficient?
Many countries will require API (Advanced Passenger Information) for passengers prior to departure or arrival. This includes passport number.
Although this is not required at the time of booking tickets, it is required before checking in. This can often be done over the counter or through online checkin.
If you are travelling to one of these countries, you can still use online check-in where available, provided that all of the relevant passport information for you and anyone travelling with you has been completed. If any passport information is missing or incorrect, you can add or edit that information through the online check-in system.
China is a country on the list that requires API.
USA is a country that will require additional information.
Airlines themselves don't need your passport number, they only need something to identify you. Most domestic flights in many countries won't even require you to have a passport.