On December 14th 2021, Mr. Wakamiya Kenji, Minister for the World Expo 2025, had a press conference and announced that there are 3 countries newly confirmed their participation in the Expo; Republic of Austria, Sultanate of Oman, and Kingdom of Tonga . This makes in total of 67 countries and 5 international organizations that have confirmed their participation, as of December 14th 2021.
For the details, please check the list of participants on the website of the Ministry of Foreign Affairs.
Ministry of Foreign Affairs "Confirmed Participants for the Expo2025"
https://www.mofa.go.jp/ecm/ec/page22e_000942.html