Not always. Saudi Aramco went public with only 1.5% of their value, and that was 25 Billion dollars, a good amount by any measure, specially useful when you need $$$ for weapons for your war in Yemen.

Besides the money, they also wanted to test the waters before going in with a larger percentage and I don't know if the market could have handled a new 1.7 Trillion dollar company joining all at once.

https://www.investopedia.com/what-is-saudi-aramco-4682590