They enter in pairs because Gandalf tells them to do it. They don't want to go in all at once, so Beorn will think they were trying to kill him
The dwarves must enter Beorn's house in pairs so that, as Gandalf tells their tale to Beorn, each entrance does not seem very imposing and Beorn's interest in the story is built up. Then the group will be able to stay with Beorn.
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At first (to the reader and Bilbo) it was to not burden Beorn with a large party too quickly. However, while that is true, it is not the main reason. Gandalf called for the dwarves so that the story he was telling Beorn would be interrupted again and again. This led Beorn to become more interested in the story, and effectively finish listening to the story before allowing the company to stay. If gandalf had everyone show at once, beorn may have sent them all off as if they were beggars.
This is a similar tactic Gandalf used with Bilbo at the beginning of the book.
A hobbit barrow is referred to as a 'smail.'
He left without a hat or a pocket handkerchief.
Bilbo demonstrates selflessness multiple times in "The Hobbit," most notably when he offers to take on the role of burglar for the dwarves despite his initial reluctance and fear. He also risks his own safety to save the dwarves from spiders in Mirkwood Forest and from imprisonment by the Wood-elves. Additionally, Bilbo shows selflessness when he spares Gollum's life during their encounter in the dark caves, choosing not to kill him despite the potential threat.
Bilbo Baggins' Diary Entry Dear diary, Today, after having thirteen unexpected guests over at my house I was taken on an adventure. I was only just starting to realize how far this journey would be and how much respect I would lose from my community when we were faced with three nasty trolls. Their names were Bert, Tom and William. They were complete morons. They were also very hungry and thought the dwarves would be perfect for dinner. Thankfully, they had no interest in eating me. Just as the first of the dwarves was about to be cooked, Gandalf made his move. Like me he had noticed that the trolls were not very smart. So when they were not looking he sneaked into the bushes behind them. Then he started teasing them but did it in a way that made the trolls think it was them. Once they were confused and had started fighting with each other, I slipped on my invisibility ring and set the dwarves free. After that Gandalf turned the trolls into stone and we had a lovely dinner. Although it was delicious, it was not a good start to our quest.
they are looking for a 14th member for a quest to kill the awful smaug(the dragon)