Assuming you're referring to "2AM" (the K-pop ballad group), here's a quick rundown:

2AM (투에이엠)

Who they are: A South Korean vocal group formed under JYP Entertainment (and later Big Hit Music for some activities), debuting in 2008. They are one of the two subgroups spun off from the group One Day (the other being 2PM).

Members:

• Jo Kwon (leader)
• Lee Chang-min (Changmin)
• Lim Seul-ong
• Jeong Jin-woon

Known for:

• Their emotional ballad style — often called one of K-pop's premier vocal groups
• Hit songs like "This Song", "Can't Let You Go Even If I Die", "I Wonder If You Hurt Like Me", and "Regret"
• A long hiatus (they were inactive as a group for years while members pursued solo careers and military service)
• A 2024 comeback with the mini-album One Take and the single "No Good in Goodbye"

"No.1" — If you mean their song or album titled No.1, 2AM released "No.1" as a track (and it's also associated with their 2010 album Saint o'Clock era releases). Let me know if you meant a specific release and I can give more detail.