given that the vast majority of modern buildings are designed with a 30-40 year lifespan in mind, not much. although i don't think that's necessarily the right question to ask. say humanity is brought down to 50,000 people tomorrow: huge swathes of infrastructure will fall apart very quickly, the electricity grid is fucked almost immediately without people to maintain and balance it, and with that goes a lot of our water and sanitation - but as long as at least some of the surviving 50k aren't dumbasses, setting up ad-hoc systems for that small a population won't be difficult. and the sheer amount of available ready-processed resources and documentation just laying around the entire planet would give a rebuilding civilization such an unreal headstart. sure, they're going to lack the manpower and expertise to maintain something like our current deal, but they're going to be and stay an industrial society so long as they don't die out.
Australia