As she has done for the canon of Sacred Scripture and for the doctrine of the faith, the Church, by the power of the Spirit who guides her "into all truth," has gradually recognized this treasure received from Christ and, as the faithful steward of God's mysteries, has determined its "dispensation." Thus the Church has discerned over the centuries that among liturgical celebrations there are seven that are, in the strict sense of the term, sacraments instituted by the Lord.
Source: Catechism of the Catholic Church (Vatican.va)