There is more than enough to keep everyone oxygenated if the worst should occur.

Also, although an immediate descent is what will happen 99% of the time, there are also certain geographical areas, especially flying east from the UK once past Europe, that will require the aircraft to remain at a higher altitude for a longer period due to terrain. So, all aircraft take that into consideration, including the 767 which also carries additional portable oxygen sets due to limitations of the chemical generators.