I usually take two, but there have been exceptions over the last nearly thirty years of gigging. When I was playing the really big places supporting touring acts, our set had need for six strings a few times. So I had bass, spare bass, sixer, spare sixer on those occasions. (and yes, I did break a string once and also once my bass just quit mid gig for no apparent reason.)
Once an old band I played with did a reunion gig, 500 tickets sold out, nice big place, only a few miles across the city, so I took everything. About seven basses, mostly 4 figure plus, my entire trace stack and a dedicated kickback monitor combo, all in the keyboard player's car, because I couldn't drive then.
Two songs in, he had a heart attack on stage right behind me and ended up being whisked away to hospital (he's fine now btw, this was over a decade ago).
I ended up staring at the mountain of gear I was stranded with, and the only solution anyone had was, 'You'll just have to leave it in his (the stricken keyboard player) car overnight, or until someone from his family can come and drive it back to yours. (It wasn't an entirely savoury part of town, it would've lasted about five minutes!)
Now I only take two! (And can drive!)
(For those that are interested, the guy's dad came back from the hospital much later that night after the gig had finished (we had to play on without him) and drove me back to mine, even though his son was in hospital. - What a legend!!)