The number is unlikely to be zero, but the matter is more complicated because some people will be working part time, whilst retired from their ordinary job, others may have two or three different jobs (like my plumber who is also working as a biochemist -true story!). And, because we are talking about the future, we should also take into account novel ways of market and job development, such as some futuristic suggestions that societies can exist without money: http://hplusmagazine.com/2014/03/26/beyond-money/