1. It really depends on what kind of games you'll be playing.NL? PL? Cash? SNG? MTT?
2. Also depends on what kind of style you have. TAG? LAG? Rock? CS? etc?
3. How much discipline do you have? How well do you control tilt? How much patience do you have?
4. What is your threshold where you can feel comfortable playing with?
5. How many tables will you be playing?
These are just few that you need to consider before considering how many buy-ins you plan to have. Don't just listen to people saying out numbers without solid reasoning.
Every pros will have different numbers. I personally took Dusty Schmidt's advice and started with 100 buy-ins but later, made it 120 buy-ins, while my threshold being 100. Anytime I only have 100 buy-ins, I'll just move down to rebuild the 20 buy-ins I lost while studying my
hands of why I lost 20 buy-ins. I am a LAG player with decent/poor tilt control and discipline from time to time that I need room to make up for my leaks and mistakes. Everyone is different and it is important to reflect yourself and to know yourself what kind of player you are before making bankroll management that suits your discipline and style. I will get criticized of my bankroll management that it's extremely nitty and I won't move up easily, but I don't mind moving up slowly because I feel the need to master 100 buy-ins of the stake I currently at to prove that I mastered the stake before I move up and to prove that I've fought through the variance to do so. I played many tables between 8-12 tables, so my mind had to be extremely comfortable where I can laugh off 12 buy-in downswing as it's a really common downswing. Good rule of thumb regarding style is, looser you are in the table, tighter your brm should be by having more buy-ins. tighter you are at the table, you can afford to be looser with brm by having less number of buy-ins and put more at stake.
Some players are very solid, playing TAG, while having good tilt control, patience, and discipline who don't donk off easily and play selectively that they can manage with 20 buy-ins. I personally can't. I've gone broke several times with 20 buy-in brm. But the moment I studied up the game and took Dusty's advice through the book Treat Poker Like a Business, I've been doing well. Of course, all of this was before Black Friday. But my logic and advice still stands.
Bankroll Management doesn't make you a winning player but it'll prevent winning player from going broke.