The pbp website preparation page suggests carrying 5 to 8 spokes.
Really?
I think I've had 3 spoke failures in many years. Never more than one on the same ride. I I think one spoke is a reasonable precaution 2 would be very cautious more than that suggests you doubt the reliability of your equipment and should probably a dress it before starting.
The preparation page is not PBP advice: they've reproduced the advice of a Chilian randonneur, Juan Salinas - I don't know wether this is to distance themselves from the advice or endorse it.
Carrying 5-8 spokes does sound a very pessimistic approach, on a foundation of doubt in the state of one's wheels. Perhaps we'll see an increase in spoke failure through fatigue (as opposed to physical damage by collision or chain action) as disc brakes increase in use and the longevity of rims rockets. Up till now mostly rims will go before the spokes reach failure through fatigue. So it's reused spokes (in a rim replacement) that finally let go rather than spokes as 'old' as the rims to which they're laced.
For a pair of normal wheels (rim brakes, 32+ and 3 x) the spoke lengths are often close enough not to need to carry different lengths. Looking up mine on
https://www.cyclebasket.com/info/common-spoke-lengths.phpa 296mm length would do for front and rear (both sides). Looking at the table one can see a lot of combinations where a single spoke length would do (if discrepancy go short). I carry a spare spoke on long rides when I've got the saddle bag or the frame bag fitted. Besides doing spoke work that length of metal might be useful for other jury rig repairs.
If a rider on PBP or indeed another long audax that they want to finish has a spoke fail, surely most will stop, remove the spoke if they can (or twist it to another), true the rim as well as possible, open the calipers a bit, and carry on, to the finish or to a bike shop (obviously depends on the time of day/night and day of week). I'm (probably falsely) assuming that everyone carries a spoke key in their tool/spares kit.
If they're carrying a spoke as well, then all but the right rear ones can be replaced at a convenient stop almost as quickly as removing, repairing and refitting a tube. Of course the spokes one can't replace without extra tools on the rear right side are those most likely to go, not because of increased stress: spokes fatigue at the same rate whatever the load (theory, puts head over parapet perhaps, especially in such esteemed company) but because they've more likely to have sustained damage (proximity to RD and chain).
