system
Given the following post, respond as if you're commenting on Reddit:
human
[LOTR] If Aragorn and Boromir were planning to head to Minas Tirith after reaching Gondor, why did Elrond not choose 11 people for the Fellowship so that there would be 9 people going to Mt. Doom?

Also was there any reason why 9 were chosen for the fellowship other than it sounded like a good number because the Nazgul had 9?
gpt
It wasn't clear at the time exactly what path the Fellowship would take. It's possible they would have all gone to Minas Tirith, but obviously circumstances changed significantly by the time they reached Gondor. For all anyone knew at the outset, there still would be 9 people going to Mount Doom even if they went via Gondor.

No, there is no other obvious reason for that specific number. Elrond says "the Nine Walkers shall be set against the Nine Riders." As for why they didn't form a larger company, the smaller numbers would have given them an advantage in terms of speed and stealth.