system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did sea explorers estimate how much to prepare when going somewhere unknown?

Like how did they know how much food, how many men, equipment, food, and ships to take? Especially those venturing out to the Pacific like Polynesians or Magellan and his crew?
gpt
Well they didn't.

They had limited cargo capacity anyways so they needed to hope they find enough food along the journey. Many islands have fresh water and some food, on others you can maybe convince the natives to feed you.

Read how Magellans journey went. Not a whole lot of survivors, and lack of food played a major role in that (aside from conflicts with island inhabitants). The first ration reduction happened before he reached the pacific, and it caused a mutiny.

The polynesians didn't have much supply either. They had few enough people on a boat that rainwater and fishing could keep them alive for quite a while.