How many times have you needed money immediately in your life, and how much money have you needed for those incidents?

Well, if you want to approach this properly... :-)

...then you'll need to evaluate the probability density of situations in your life where not having a certain amount of cash on hand will lead to severely negative outcomes (aka high costs). I expect that you'll have much difficulty in trying to form a reasonable estimate (see Nassim Taleb and the general Black Swan concept). Notably, limited amount of historical data (as in, e.g. your personal experience) is not all that good a basis for estimations.

There is also a whole bunch of other factors in play -- do you have kids? do you travel much? outside of the US? etc. etc.