I have worn loose fitting ExOfficio boxers (NOT the boxer brief style) for four Caminos. They are now my daily wear, here in South Florida. At the risk of TMI, let's just say they are (IMHO) the most comfortable thing you can wear with cargo type shorts...also my daily uniform...
For a month-long Camino, three pair should be sufficient. One on you, one washed last night, and one washed two nights ago. This cycle works very well.
I found, as many other do, that the "three-set model" works well too. Some folks drop it to two total, one on and one off. But that presupposes a good place to do and wash and dry the shorts, socks, and shirts daily. On the Camino, it DOES rain fairly often.
Originally, I started doing Caminos carrying four sets. But that extra set of shirt, shorts, and two pair of socks cost me one kilo. The extra set got sent down the road to Ivar.
Now, for a two week or less Camino, I use this "three set" model with ONE pair of zip off cargo pants. After two weeks, they tend to get a bit funky, even if I wash the bottom legs as needed to remove mud and road grime.
For a month-long Camino, I will typically put an extra pair of sip-off cargo pants in a ziplock bag, at the very bottom of my rucksack. This all works FOR ME.
I hope this helps.