It is difficult to know for certain, because most people do not make their mortal sins public. However, it is likely that many of the saints did commit mortals sins even after their “conversion” to a holy way of life.
In causes for canonization, the Church does not look for perfect people (which—except Jesus Christ and the Blessed Virgin Mary—do not exist) to hold up as models, just persons who have shown virtue to a heroic degree (and, evidently, who have persevered to the end in their holiness).
As the Catechism of the Catholic Church puts it,
By canonizing some of the faithful, i.e., by solemnly proclaiming that they practiced heroic virtue and lived in fidelity to God's grace, the Church recognizes the power of the Spirit of holiness within her and sustains the hope of believers by proposing the saints to them as models and intercessors (no. 828, emphasis added).
The following document shows the steps in the process for canonizing a saint: https://www.ewtn.com/johnpaul2/cause/process.asp.
As can be seen, in the documentary phase, the Postulation (the commission established by the diocese promoting the cause)
must gather testimony about the life and virtues of the Servant of God. Also, the public and private writings must be collected and examined. This documentary phase … concludes with the judgment of a diocesan tribunal, and the ultimate decision of the bishop, that the heroic virtues of the Servant of God have or have not been demonstrated (op. cit.).
Hence, an occasional lapse—even a grave one—is not necessarily an obstacle to canonization, provided there is sincere repentance and a tendency to virtue and holiness of life.