A differing number is sometimes seen as a Protestant vs. Catholic thing, but in reality, the number isn't the point.
St. Paul in his letter to the Galatians lists specific “fruits” of the Holy Spirit, “the fruit of the Spirit is love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control; against such there is no law” (Galatians 5:22-23 RSV-CE).
The Catechism of the Catholic Church defines this fruit as “perfections that the Holy Spirit forms in us as the first fruits of eternal glory” (CCC 1832). Basically, if we are living a “life of the Spirit,” these “fruits” will be found in our lives.
Most translations of the above passage only list nine fruits, while the Latin Vulgate provides a list of 12 fruits, adding modesty, generosity, and chastity.
But the fruit of the Spirit is, charity, joy, peace, patience, benignity, goodness, longanimity, mildness, faith, modesty, continency, chastity. Against such there is no law (Galatians 5:22-23 Douay-Rheims).
According to the Catholic Encyclopedia, “there is no doubt that this list of twelve — three of the twelve are omitted in several Greek and Latin manuscripts — is not to be taken in a strictly limited sense, but, according to the rules of Scriptural language, as capable of being extended to include all acts of a similar character.”
With this in mind, the Catholic Church has accepted these “extra” fruits and lists them in the Catechism, “The tradition of the Church lists twelve of them: ‘charity, joy, peace, patience, kindness, goodness, generosity, gentleness, faithfulness, modesty, self-control, chastity'” (CCC 1832).
St. Thomas Aquinas defends this expanded list in his Summa Theologiae.
The number of the twelve fruits enumerated by the Apostle is suitable, and that there may be a reference to them in the twelve fruits of which it is written (Apocalypse 22:2): “On both sides of the river was the tree bearing twelve fruits.” … As Augustine says on Galatians 5:22-23, “the Apostle had no intention of teaching us how many [either works of the flesh, or fruits of the Spirit] there are; but to show how the former should be avoided, and the latter sought after.” Hence either more or fewer fruits might have been mentioned.
At the same time, the Catechism also cites nine fruits of the spirit in a different paragraph, “By this power of the Spirit, God’s children can bear much fruit. He who has grafted us onto the true vine will make us bear ‘the fruit of the Spirit: … love, joy, peace, patience, kindness, goodness, faithfulness, gentleness, self-control.’ ‘We live by the Spirit’; the more we renounce ourselves, the more we ‘walk by the Spirit'” (CCC 736).
In reality, the exact “number” of fruits is not something we need to focus on. Christians are called to live a life of the Spirit, producing much fruit that is shown through our thoughts, words and deeds. This is the meaning that St. Paul is going for, as he spells out the nine characteristics as, simply, fruit (singular) of the Spirit.
Though the difference in number and lists might make memorization hard, it also is a reminder of God’s real message to us.
As Jesus told his disciples, “By their fruits you shall know them” (Matthew 7:16).