Question about buy-ins.

When someone says to use 20 buy-ins for example for cash games do you need to ALWAYS have at LEAST 20 buy-ins? or just start with 20?

hope you understand my question.

This is usually can be said regulars who beat limit where played, they do not need a big bankroll, enough to have 20-40 bi, almost zero chance that they will all lose.

But for new players definitely need at least 100bi, because they have a better chance of losing all the money and they are more likely must to go down on lower limits, if they start to lose money.

It's a threshold - Meaning if you have less than 20 buy ins you need to drop down stakes to maintain 20 buy ins or more. 20 buy ins really isn't enough IMO. 40-50 seems better to me. To answer your question, yes - always have the given buy in amount to manage your bankroll correctly.

Always. You need to be able to afford the game to cover your losses. When people are short stacked, they tend to make hasty decisions as well.