As I read the Catechism and Scripture, the antichrist could be a single man or a group. I don't believe the Church has defined the question one way or the other, but here is the current Catechism on the matter:
Rev. 13:1 ff speaks of a "beast from the sea" having 7 heads, which could be read as an alliance of 7 following one spirit - the evil one. He/they will become the godless evil secular dictator over all sometimes identified with the antichrist. He/they will have an ally in the ecclesial world in Rev. 13:11 - the beast from the earth, "it had two horns like a lamb and it spoke like a dragon". This beast is of the same mind and heart and spirit as the one from the sea, and is identified as the "false prophet"(Rev.16:13, 19:20, 20:10) who leads the false church into the Lie. This is also identified as "antichrist", and both/all are - one secular ruler(s) and one a "church" leader.