Thank you for the question on understanding the parables, “How many non-saved people understand parables?”
In our modern age of the easy access to books, recordings or videos, I believe that many non-saved individuals can read and understand the parables. If the parable was heard the first time without anyone explaining it, the listener will have a difficult time understanding the parable. For example, he would not understand what the “seed” is in the Parable of the Sower. That person may think that Jesus was just speaking about farming without realizing there is a spiritual dimension to the parable.
The understanding of the parable is more than comprehension of the words and ideas. The understanding of the Scriptural text would include believing and applying the truth in one’s life. To know the facts or its meaning without faith and application of that truth would reflect as James says that a person who looks in the mirror and then forgets the reflection of that mirror (James 1:21–25).
Many non-Christians know or have heard about John 3:16 that God so loved the world that he gave his One and Only Son that whoever believes in hims shall not perish but have eternal life. That person may be able to explain that verse, but that person refuses to believe it. That person may have his reasons as his viewpoint may include the non-existence of God or the rejection of the Bible as being the Word of God. Knowing the Biblical facts is not the same as believing the Biblical facts.
SUMMARY: The explanation of Parables is readily available via the media to any person, but the deeper question is this, “Does the reader/listener believes the Scripture?”
For more perspectives:
https://www.quora.com/How-many-non-saved-people-understand-parables