Depends on the Player and the stakes i guess. Its about Variance. The better you are compared to your opponents the less variance you will face because of the edge you have over them.
As a general rule of thumb you want to have a minimum of 20-30 buyins for your limit in NL cash wich would be 3-5% of youre roll. If you are a regular at 200NL online and by whatever reason decide to start at 10NL again, i can see people doing it with 10 buyins, thus 10% of the overall bankroll, but even for someone who crushes a limit thats a pretty aggresive bankrollmanagement, and it has to be expected that variance will crush you some of the time.
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Poker is a game of skill against your opponents luck
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