ANSWER: It is important to note that the books of Matthew and Luke are the only two that give any significant details surrounding the actual birth of Jesus. Additionally, Luke 2 is the only place in the Bible that discusses shepherds and angels that are the focal points of your questions.
8. Now there were shepherds in the same country, who were dwelling in the fields and keeping watch over their flock by night; 9. And suddenly an angel of the Lord stood by them, and the glory of the Lord shined round about them; and they were seized with great fear (Luke 2:8 - 9, HBFV throughout unless stated)
As you can read for yourself, the Bible does not specify the number of shepherds who heard about Jesus and his birth. The fact that there was more than one is all that can be discerned from the Greek text. It is interesting that they were living out in the fields nearby.
Much of Western Culture depicts the first coming of the Lord as having occurred in December or January, which are two of the coldest months of the year in that area near Bethlehem, and MUCH too cold for the shepherds to be "living out in the fields." They would begin moving the herds to enclosures in the late fall and only take them out to pasture during the daylight hours. This also corresponds with secular history that says a census (which was the reason Joseph and Mary had to travel to Bethlehem), dictated by the Roman Emperor Augustus Caesar, was carried out in September or October.
10. But the angel said to them, "Do not be afraid; for behold, I am announcing to you glad tidings of great joy, which shall be to all people; 11. For today, in the city of David, a Savior was born to you, Who is Christ the Lord. 12. Now this is the sign for you: you shall find a babe wrapped in swaddling clothes, lying in a manger" (Luke 2:10 - 12)
How many angelic beings appeared to the shepherds? Both the English translations and the original Greek use a singular designation for "an angel of the Lord." In the first manifestation, there was only one angel. Later it was a "multitude" (KJV Bible) or a "host" of angels who appeared when Jesus came into the world.
It is interesting to note that much of Western Culture has misquoted what the host of angels shouted for joy. They DID NOT say, "Peace on earth and good will toward men." This is an erroneous version of what the King James Bible actually says in Luke 2 verse 14. What they ACTUALLY proclaimed was much closer to the following.
13. And suddenly there was with the angel a multitude of the heavenly host praising God, and saying, "Glory to God in the highest, and peace on earth AMONG MEN OF GOODWILL" (Luke 2:13 - 14)
The angels were not requesting "good will" (the Greek here means delight or kindness) come to every unrepentant sinner and enemy of God who would not appreciate the birth of Jesus. Instead, peace would come to everyone seeking the manifestation of the Messiah and who do his or her best to obey God.