UEFA secretary general Gianni Infantino said that no more than one venue per country would be permitted and that the semi-finals and final would all be played in the same stadium. The venues would be chosen in September next year, he added.
UEFA voted in December to stage the tournament in cities across the continent rather than one or two single host countries, in a radical departure from the traditional format for major tournaments.
European football's governing body said the move would allow smaller countries a slice of the action as they would not be able to stage unwieldy 24-team tournament on their own.
Host countries will not qualify automatically but if they do, their national team will be guaranteed to play at least two of the three group matches at home, Infantino told a news conference in Nyon, Switzerland.
Infantino said: "The matches of the Euro will be split into 13 packages - 12 cities who will have three group stage matches and one knock-out round match and one city will host the two semi-finals and final. There will be only one venue per country, and it means the semi-finals and final will be played in the same venue."
Istanbul is the favourite for the final - but only if it loses its bid for the 2020 Olympics.
UEFA president Michel Platini has already said he would back the Turkish city and he confirmed his position again on Friday.
He said: "That would be my vote, but if they get the Olympic Games it's out of the question that they could stage the Euros or a match."
UEFA also confirmed that two cities with smaller-capacity stadiums will be among the 13 chosen in an effort to allow smaller countries to enter the bidding - only 21 of the 53 UEFA member nations have stadiums of 50,000 or more.
Two host cities can have a stadium capacity as low as 30,000, 10 stadiums will have a 50,000 minimum capacity and four of those hosting the quarter-finals will have grounds of at least 60,000.
The stadium that hosts the two semi-finals and final will have to be able to seat more than 70,000 fans.
But when the idea was originally floated as a possibility last year, many fans were instantly critical. Complaints centred on the expected huge costs to follow a team across the continent, and also that the unique football tournament atmosphere would be sacrificed.
Euro 2020 marks the competition's 60th anniversary.
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- Gianni Infantino