That is a difficult question to answer. In the first eight or nine centuries there was no formal process of canonization. People were recognized as saints because of a kind of popular acclamation. People were believed to be saints because they had been martyred for the faith or they had lived very holy lives. Often their graves became places of pilgrimage and prayer. We have no idea of how many people’s holiness was not recognized. That is one reason why we have the Feast of All Saints.
Sometimes the recognition of holiness was particular to an area or community. Sometimes the reputation for sanctity spread beyond national borders.
It was not until 993 that the first official canonization took place. It was then that Pope John XV declared Bishop Ulrich of Augsburg a saint.
The original edition of Butler’s Lives of the Saints, published between 1756 and 1759, had 1,486 entries. The 1956 revision contained 2,565. Given the many people that Pope John Paul II beatified or canonized during his papacy (a record number) the next revision may extend well past its current 12 volumes.