The consumption of parts for repair increases as the game progresses. While there are recipes to somewhat mitigate this, they are far from sufficient (and the main Gosenberg region didn't have one: there's only one in the first Gosenberg region, as far as I understand).
This means that a the party has to leave the last region after a fairly limited number of fights to stock up on parts, as the local production in the town doesn't have any chance to keep up.

The last fight (after the community patch) required 32 parts after a fight with a regular "easy" contract bandit gang mission playing on normal mode, and before the patch each fight would consume 15-25 or so parts, which was still too much to allow for staying in the region without resupplying outside of it. While it's possible to have the town smith do the repairs, that would just mean you'd have to leave the region to trade for money (which I do in parallel with the parts resupply anyway).

A part of the problem may be that the party is somewhat under leveled for the region (level 10), which has been addressed by the patch, but harder hitting enemies are nevertheless going to destroy more armor.

In the early game, I noticed I was using a lot more repair parts than it seemed would be necessary. I'd have several members get hit but with maybe just a handful of armor points lost for some of them. It's a small chore, but I now individually right-click each portrait to see if the armor actually needs repair or if they can instead get by with a little damage for the next fight. This has really cut back on how quickly I'm burning through repair parts.

Also, if you're anywhere near a town after a battle, have the blacksmith repair everything for dirt-cheap.