Here is the reading you are having trouble with:
Mk 8:14-21To approach this reading, we should start with the literal sense. This does not mean a word-for-word literal interpretation, but an examination of the text to discover what the original human author intended to teach and how his audience understood him. We must also look at the context of this reading, and what the Church has to say about it.
The disciples had forgotten to bring bread,
and they had only one loaf with them in the boat.
Jesus enjoined them, “Watch out,
guard against the leaven of the Pharisees
and the leaven of Herod.”
They concluded among themselves that
it was because they had no bread.
When he became aware of this he said to them,
“Why do you conclude that it is because you have no bread?
Do you not yet understand or comprehend?
Are your hearts hardened?
Do you have eyes and not see, ears and not hear?
And do you not remember,
when I broke the five loaves for the five thousand,
how many wicker baskets full of fragments you picked up?”
They answered him, “Twelve.”
“When I broke the seven loaves for the four thousand,
how many full baskets of fragments did you pick up?”
They answered him, “Seven.”
He said to them, “Do you still not understand?”
In Mark, the scene just prior to this selection, Jesus has just multiplied the loaves and fishes for the crowd. He then gets in a boat with his disciples and this scene begins. Because the disciples "had only one loaf with them in the boat", Jesus uses this situation to teach them. He tells them to "guard against the leaven of the Pharisees and the leaven of Herod." Leaven, of course, is used to make bread rise. Just a little bit changes the bread. So, the disciples should beware of those (Pharisees and Herod) who are corrupt and can change them (or the crowds), for the worse.
But, they don't get it. As St. Josemaria Escriva says,
"they (the apostles) weren't educated; they weren't very bright, if we judge from their reaction to supernatural things....When Jesus uses the image of the 'leaven' of the Pharisees, they think that he's reproaching them for not having purchased bread....These were the disciples called by our Lord."They are not unlike us, who don't understand everything!
But, Christ continues to teach them, even though they may not get it all.
The 12 baskets of bread in one episode and the 7 in another are symbolic numbers.
The numbers in the Bible mean a lot more than what we might first think. Especially to Jews, numbers had deep symbolic meaning and this is the case here as well. 12 is a symbol of the 12 tribes of Israel, whom Jesus has come to save. 7 is a symbol of the 7 tribes of Canaan who lived alongside Israel and to whom the Gospel message is also intended to serve. Thus, he is telling them that as Messiah, he has a universal mission to all people.
But, the disciples still do not understand and they will continue to struggle with understanding the true nature and mission of Jesus until Pentecost.
I hope this helps.