Singing the same note is unison.
What is it called when you sing the same note?
In music, unison is two or more musical parts that sound either the same pitch or pitches separated by intervals of one or more octaves, usually at the same time. Rhythmic unison is another term for homorhythm.
What is it called when many singers sing together?
Multiple people singing together with piano or organ accompaniment is called a choir. Multiple people singing without any instrumental backup is called an acapella choir, or often just acapella. Multiple people singing and dancing, or singing with more than one instrument is called an ensemble.