I've been up to 18k, with oxygen obviously.
Once I did a 5hr flight almost entirely at 16k, UK to Slovenia, just staying on top of the muck. That used up most of a bottle (myself and my partner) and luckily I had one more so I could fly back to the UK on a similar route (over the Alps).
Aircraft performance permitting, altitude is no big deal. The portable oxygen kits are cheap - a few hundred quid. It opens up a whole new world for VFR touring in Europe, away from the UK-style PPL scud running cr*p, and is virtually essential for flying IFR on Eurocontrol-acceptable routes.
The big deal is getting them refilled. The scuba shops tend to get suspicious and when you mention "aviation" they usually leg it. Usually one can't get a refill away from home. I rent a huge welding oxygen (it's all the same stuff) bottle from British Oxygen - about £90/year rent and about £20 for an exchange bottle.