I have read books that recommend as much as 100 times the buy in. Start with tiny buy in tournaments and make certain you are beating them first before you are jumping into the big ones. You will go on long streaks of no cashes and if you are cashing but only just small cashes then you are not playing correctly. You will need to make final tables. Average players can limp into the money and make pennies. In that case it is better to play cash games.
You should be cautious about how much you spend on a particular buy-in. MTT’s are notorious for negative variance, and it’s quite common to bust out of five (or many more) consecutive tourneys without cashing once. A general rule of thumb is to keep more than enough buy-ins in your account to handle the inevitable variance that goes hand-in-hand with large player fields in online tourneys.
Im in the same boat.. I think ive finally found a game that suits my style.. thats what I find is causing me issues.. i play too many different types of mtts and tourneys... ive now started focusing on one type of mtt until my results and my confidence reach a happy medium. Thats what I would suggest.. Find your range and your type of game and own it!
At least 100 buy ins. For large field tournies, maybe even more. People generally really underestimate the value of good BRM. Like its sooo important to play with a nitty bankroll. Just the psychological impact of losing 5% of your bankroll in a day is so much tougher than losing 1% of your bankroll in a day, and that impact could effect your play.