For England, there are 16 possible R16 opponents at the moment. In short:Thanks. I must be a seriously boring person as I found that interesting! I thought I'd heard Austria were likely opponents but clearly they're not, they're just one of many.
Finish 1st, play 3rd place team from group D, E or F*.
Finish 2nd, play winner of Group A.
Finish 3rd & go through, play winner of group E or F*.
*which one depends on which groups the four 3rd place teams come from. There are 15 possible combinations of 3rd place groups.