Question: How many teams perform in the UEFA league group that precedes the knockout phase?

Context articles: 2016–17 UEFA Youth League knockout phase
2009–10 UEFA Europa League knockout phase
2000–01 UEFA Champions League
2010–11 UEFA Europa League knockout phase
2016–17 UEFA Europa League knockout phase
2014–15 UEFA Champions League knockout phase
2017–18 UEFA Champions League knockout phase
2016–17 UEFA Champions League knockout phase
2017–18 UEFA Champions League group stage
2014–15 UEFA Europa League knockout phase

The 2016–17 UEFA Youth League knockout phase (play-offs and round of 16 onwards) began on 7 February 2017 and concluded on 24 April 2017 with the final at Colovray Stadium in Nyon, Switzerland, to decide the champions of the 2016–17 UEFA Youth League.
 A total of 24 teams competed in the knockout phase.
The knockout phase of the 2009–10 UEFA Europa League began on 18 February, and concluded with the final at the Volksparkstadion in Hamburg, Germany on 12 May 2010.
 The knockout phase involved the 24 teams that finished in the top two in each group in the group stage and the eight teams that finished in third place in the UEFA Champions League group stage.
The 2000–01 UEFA Champions League was the 46th season of the UEFA Champions League, UEFA's premier European club football tournament, and the ninth since it was rebranded from the "European Champion Clubs' Cup" or "European Cup".
 The competition was won by Bayern Munich (first title since 1976), who beat Valencia 5–4 on penalties after a 1–1 draw after extra time.
 It was their first UEFA Champions League title, and their fourth European Cup title overall, it was Valencia's second consecutive final defeat, losing to Real Madrid in the previous season.
 The knockout phase saw Bayern eliminate the preceding two Champions League winners, Manchester United and Real Madrid, winning all four games in the process.
 Valencia, meanwhile, defeated English sides Arsenal and Leeds United in the knockout phase en route to the final.
The knockout phase of the 2010–11 UEFA Europa League began on 15 February and concluded on 18 May 2011 with the final at Aviva Stadium in Dublin, Republic of Ireland.
 The knockout phase involved 32 teams: the 24 teams that finished in the top two in each group in the group stage and the eight teams that finished in third place in the UEFA Champions League group stage.
The 2016–17 UEFA Europa League knockout phase began on 16 February and ended on 24 May 2017 with the final at Friends Arena in Solna, Sweden, to decide the champions of the 2016–17 UEFA Europa League.
 A total of 32 teams competed in the knockout phase.
The 2014–15 UEFA Champions League knockout phase began on 17 February and concluded on 6 June 2015 with the final at Olympiastadion in Berlin, Germany to decide the champions of the 2014–15 UEFA Champions League.
 A total of 16 teams competed in the knockout phase.
The 2017–18 UEFA Champions League knockout phase will begin on 13 February and end on 26 May 2018 with the final at the NSC Olimpiyskiy Stadium in Kiev, Ukraine, to decide the champions of the 2017–18 UEFA Champions League.
 A total of 16 teams compete in the knockout phase.
The 2016–17 UEFA Champions League knockout phase began on 14 February and ended on 3 June 2017 with the final at Millennium Stadium in Cardiff, Wales, to decide the champions of the 2016–17 UEFA Champions League.
 A total of 16 teams competed in the knockout phase.
The 2017–18 UEFA Champions League group stage began on 12 September and will end on 6 December 2017.
 A total of 32 teams compete in the group stage to decide the 16 places in the knockout phase of the 2017–18 UEFA Champions League.
The 2014–15 UEFA Europa League knockout phase began on 19 February and concluded on 27 May 2015 with the final at Stadion Narodowy in Warsaw, Poland to decide the champions of the 2014–15 UEFA Europa League.
 A total of 32 teams competed in the knockout phase.