Commerce: Extra information per quantity bought Is there any module or solution that allows extra information to be collected for each item bought? Example of what I mean below:

A user can buy any number of tickets, and for the amount of tickets purchased they have to complete details of the other guests (name, email address etc...). So if they buy 10 tickets they have to complete 9 lots of guest information in addition to their own.

Is there any module that can facilitate this? Additionally, the user should be able to log in and change the details of the guests before the event occurs. This part I'm almost certain will be a custom module, but I'm open to any ideas!
Not a simple answer here, you've got a lot of ways to do this, but if you're focusing in the ticket/registration case, you could take a look to Commerce Registration, that will provide the closest feature I know for that matter.

It is based in Entity Registration, so you've got there a starting point.