The people living in the town with the healed demonized man were terrified of Jesus. So sending the man out to spread the news wasn't going to disrupt Jesus' ministry as he is about to move on to the other side of the lake. In other stories, we find people wanting to make Jesus king when they witness His power (instead of being afraid, the people think they've now got someone who can kick the Romans out of power). So asking the family to keep what had happened a secret would prevent such a commotion among the people in Capernaum and allow Jesus to continue doing His ministry without the people demanding Him to become king.
Thank you for your question. Please see the response to the question above, as it is a similar question.
The number 12 does have significance in the Bible. In this story, though, I don't know that the number has the significance on the same level as the 12 tribes of Israel or 12 apostles. I think Mark is letting us know a detail about the life of the woman and the young girl so we know they are real people (as opposed to made up characters).
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