Platini said that the format would continue to involve six-team groups, however up to three teams per group could qualify for the final tournament. Hosts France will qualify automatically.
UEFA has 53 national associations while the British colony of Gibraltar has been accepted as a provisional member.
Platini said he was convinced the 24-team tournament would work but pointed out it was not originally his idea.
"This decision was made by the executive committee and Congress in 2006 following a request from Ireland and Scotland," he told a news conference on Thursday following a UEFA executive committee meeting.
"I thought about it a great deal to see if the quantity would impose itself on the quality but I think we have 24 good teams in Europe, more or less of the same calibre, so the quality won't be affected.
"The only small difficulty will be with the qualifiers, because there will be some less interesting matches at that level as the national associations wanted the same system.
"We will have two or three teams qualifying from groups of six, so matches will be less decisive, but the tournament itself will be just as interesting."
The Frenchman spoke of the importance of allowing as many teams as possible to experience a major tournament.
"It is very important. Imagine what would happen to the following of football here if Bulgaria never made it to a top football tournament.
"It wouldn't be good for football, it wouldn't be interesting for your fans and audience."
Earlier, UEFA confirmed the bidding regulations for Euro 2020, which will be staged in 13 different countries around the continent.
UEFA said the bidding phase would be officially launched on April 26 and that the deadline for the formal confirmation of bids would be September 11.
Bid dossiers would have to be submitted by April 25 next year and they would be evaluated between May and August. The host cities would be chosen by UEFA's executive committee in September 2014.
