Just check, but your outfitter should have a loaner available in case of some mishap.
This.
Right, so I'd have to say "it depends". Plains Game? Well, I KNOW my PH has a spare rifle. And perhaps I'm not as concerned about PG since the ranges are likely from 70-300 yards.
But DG? Yeah, I darn well might just bring a spare rifle.
In all my years of shooting (appx 35 years...and I'm 44 years old) I've never had a rifle fail me. Sure, I've had a few mis-fires...but a rifle and and unto itself fail me? Nope. I've used Rem. 7's, Win Pre-64's, many thousands through a Rem. 541...I've had my revolvers screw up (ejector rod back out causing some agony over getting that fixed in the field) but...
Nope. Never had a rifle fail. I mean, FAIL. As in, broken firing pin...linkage, whatever.
In the end this is all a game of "cost vs. consequences". My last deer hunt I brought one rifle, five rounds. Yep, my Merkel K1 and just 5 rounds. Granted, I grabbed the wrong box of ammo when I left but still. I fired one round, dropped a pig. End of story. Rifle did just fine.
If I'm going to Africa (gee, isn't that what this site is about?) and I'm hunting PG, well, my first question to the outfitter is whether or not there is a back up rifle available. Crap happens, rifles get lost, ammo gets lost. Is there a contingency to handle it? Generally, yes.
But DG? That kind of changes the game. The need for familiarity with ones tools is critical to success...and in some cases survival. I have a Montana 1999 in .375 H&H in the safe...but if I was going on a DG hunt I darn well might pack a spare rifle in case something went wrong with my Montana. Sure, that Montana has never misfired or failed but it could happen. The real question is whether or not I'd like to face DG with an unfamiliar rifle.
In that case, nope. i'd likely want a familiar back up. On the other hand that brings to question if I actually was familiar with a rifle enough to trust it.
I suppose in a perfect world the ultimate answer would be two identical rifles of the same caliber. No need to swap ammo or learn anything new. Just a perfect back up.
Yet the world isn't perfect.
So, for today, I have one .375 H&H...no back up. It's claimed one deer. And fired about 400 rounds. I'm confident it will work when needed. Of course, Life is what happens when planning goes to pot...