How many writings of those who were disciples of the first 12 apostles (including Paul) are available today? I guess, no need to include Mark and Luke as their writings are included in the Bible.
The group you're referring to is known as the Apostolic Fathers. The exact number is unknown, because some writings cannot be dated precisely.
These writings we can say with some confidence are from the Apostolic Fathers:
These writings may or may not belong to that generation: