Buy the biggest card that will hold a full day's shooting. Much better to have all your eggs in one safe, secure, basket than to try and juggle three or four baskets of eggs.
I've never lost an image from memory card failure in over 15 years of use. I have, however, lost images by losing a memory card. I've also seen my friend turn puce as she realised that she'd just pressed 'Format' on a card full of unsaved images, rather than the blank card she thought was in the camera.
SD cards, in particular, are fiddly little things. It's very easy to drop one while swapping them over. It's very easy for them to slip out of a pocket. It's very easy to put one 'somewhere safe' in the heat of the moment and forget about it totally.
But having just one card, that will last all day, means that the only way you can lose the card is to lose the camera. Yes, if the card should fail (never happened to me) then you've lost a lot of images - assuming that the numerous card recovery programs can't bring them back. But using more than one card multiplies, several-fold, the chances of losing some of your images.