Why did Erza challenge that many monsters at the Pandemonium? During the Pandemonium contest at the Grand Magic Games,


   Erza challenges all the 100 monsters at once, but she could have won the full 10 points by challenging and beating "only" 51. 


Why did she do it?
During the Phantom Lord arc, Erza herself mentioned that Fairy Tail is not concerned about competition with other guilds. As Natsu told Sting during the Chariot competition, they entered the Grand Magic Games for their comrades, and to prove that Fairy Tail will keep going forward. They wanted to win the Games to regain their guild's lost respect.


   As such, the Pandemonium contest was more than just winning 10 points for Erza. If she had only challenged 51 monsters, it was possible that she would get to battle mostly the weaker monsters, and someone else might beat the S-rank and A-rank monsters. 


That would not earn them any respect, but people would just consider that she was lucky. Erza did not want to take that chance.

Also, Fairy Tail from 7 years ago was known for going overboard and doing things rashly. Doing it differently now would not convince people that Fairy Tail is back, but only create a perception that they have become weaker and are lacking in confidence.