As with everything else in poker "it depends". Tournaments with large fields have higher variance than those with small fields. The higher your edge, the easier it will be to pull out of any downdraft. It also matter, if you are willing to move down, if you lose, and how high your tolerance of swings is. Or if maybe the "bankroll" is not really a bankroll, because you are able and willing to top it up with money from salary or other sources if needed.
Finally it matter, if you play the same games or mix it up. If the latter case your average buyin is more important than the highest one. But of course there is a limit to, how far you can take that concept. If you play a $109 MTT and 99 $1,1 MTTs, your average buyin is 2,18$. But variance is going to be way larger than, if you had played 100 $2,2 MTTs.
To make a long story short for me personally it has always worked well to use a simple 100X rule for MTTs, meaning that the highest, I will buy in for, is 1% of my bankroll. But I will also play cheaper events, so my average buyin is for less than 1%. But if someone always play large field events, and dont want to see massive swings in their bankroll, it can be reasonable to be more conservative than this.
