You don't intuit it, you calculate it. If you claim to be able to predict coin flips with a specific accuracy I can give you the exact number of trails N you ne

You don't intuit it, you calculate it. If you claim to be able to predict coin flips with a specific accuracy I can give you the exact number of trails N you need to be x% confident. If you claim 90% accuracy and I want to be 95% confident with the standard alpha of 0.05 you need 38 trials.