How many event cards are enough?

I suggest that it depends a ton on the game.

The general rule of thumb I go by -- in the hopes of being published -- is "use as few components as possible to achieve the desired effect." The same would be true of self-publishing: no point in driving the cost of the game up if there's no reason to.

So that provides two guiding principles: use a few cards as possible, and use every card you need to make the game work right.

Are your event cards all unique? The pros of unique events are that they'll make every game different. The cons are that the game could be too chaotic, making it so difficult to reasonably guess what will come next that strategies are constantly invalidated.

If the event cards aren't unique (there are x cards that do this, y that do that, etc.) then how important are the one-shots that don't get discarded? That is, would the game still work if, for example, you had 45 cards and you reshuffled the discarded ones for the last quarter of the game? That would mean there would be no one-shots at all towards the end.

Do the event cards vary greatly in "power" or effect? If they do and you want the more powerful ones to be less common then you'll need plenty of less-powerful cards to create an ideal ratio. If you want a lot of different powerful cards that are rare then that means lots more less powerful cards.

All of these things and more will affect the ideal number of cards. There's no "one" answer.

Hope that helps.

-- Matthew