On May 10th, Minister Wakamiya Kenji had a press conference and announced that Mongolia has confirmed its participation in the Expo. This makes in total of 106 countries/ regions and 7 international organizations that have confirmed their participation.
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