For example, ABCD team 4, A wins B, C wins D; In the second round, A wins D and B wins C; Three rounds of c wins a, b wins d.
Then the result is two wins and one loss for ABC and three losses for D. Even if you get six points, there is still a team that wants to rank third and faces being unable to play. (In this case, the scores of all teams add up to 18, which is also the case where all teams score the most. The total score of winning a game is 3+0=3, and the total score of drawing a game is 1+ 1=2. The total score is the largest. In the six group matches, 18 was the biggest total score. The real situation is that in teams with the same score, the outcome is decided according to the goal difference or the number of goals scored.
If a team scores 7 points, it means two wins and one draw. Assuming that this team still can't guarantee to play, then there must be two other teams whose scores are greater than or equal to 7, so the total scores of these four teams are greater than or equal to 7+7+7+0=2 1, which exceeds the highest score 18 in all cases. It is impossible and untenable, and getting 7 points is bound to qualify.
So the answer is 7 points to ensure qualifying.