Does maximum altitude increase when pilot is acclimatised? Normally 10,000 ft is considered safe altitude for emergency descents and 12,000 ft a maximum that can be flown in unpressurised aircraft without using supplementary oxygen.

But when flying in the mountains with a crew that normally lives at say 5,000 ft or more, can they afford higher altitude? And by how much?

Note: The 12,000 ft I've seen somewhere, but I don't know whether it is actually a regulation or just recommendation.
Yes, maximum possible altitude at which a pilot can fly an Unpressurized Airplane without the need for supplementary oxygen does increase with acclimatisation. But the international aviation laws do not take this into account and hence they have a general altitude limit for unpressurized flying without supplementary oxygen.