It depends on your ABI (average buy-in). I wouldn’t start Bankroll management with less than 50 buy-ins at your average stake. 100 is just to be safe. Don’t be afraid to play at the lowest available stakes if you don’t have a lot of money. It’s better to lose less at low buy-ins as a beginner than to jump straight into high-stakes games where you’re an underdog against stronger players from the start.
As a beginner, don’t overexert yourself by playing eight tournaments at once. Stick to one or two windows so you have enough time to think about your decisions and pay attention to what’s happening at the table.
Invest in tracking software/HUD and learn to interpret the numbers it gives you about your opponents. This means less focus on specific opponents because it records hand data even when you’re not in the hand, and this data can be used to make faster decisions if used correctly.
If you’re not winning yet, it doesn’t matter what your Bankroll management is. I highly recommend studying as hard as possible and playing at very small stakes and freerolls (some field adjustments, such as slightly underbluffing, can help). Once you’ve mastered the material and at least broken even, the old story of 100 buy-ins is a good starting point, BUT the math is much more complex than it seems (field sizes, payout percentages, tournament type, etc. – everything matters).