There are 6 groups of 4 teams = 24 teams.
In order to get down to 16 for the knockout stages, the top 2 from each group go through, plus the 4 best 3rd places out of the 6 (so 2 teams finishing in 3rd place won't go through).
NI have qualified as Albania have finished 3rd in group A with 3 points and a goal difference of -2 and Turkey have finished 3rd in group D with 3 points and a goal difference of -2. NI have 3 points and a goal difference of 0, so they're guaranteed to be one of the 4 best 3rd placed teams, irrespective of the games yet to be played in the final 2 groups .... although the results of the remaining games will determine who NI meet in the next round.