spare spokes
 

I think that spokes are only sold in sets (i.e about 36 spokes + nipples). They are very much bike and model specific. Since on a long trip your bike is most likely heavily rear weighted I'd recommend you to carry at least the rear spoke set. If there is metal fatigue they might fall like dominoes i.e. a few neighboring ones at one go (seen this happen in rough terrain). Also remember to check their tightness every few days - the loose ones put extra stress on the tight ones.

Its usually your front wheel that takes most of the abuse. The rear ones losen more quickly under load but its the front wheel that gets knocked about really.