At least one! 
Just a joke, but I think 2 is a good number to start with. One is fine, but if it has a problem, you don’t have a backup hive. A second hive can provide you with extra bees if your other hive is weak, or honey and pollen stores if it is low on food.
More than 2 hives might get a bit overwhelming for a beginner. I once had 5 hives - it was pretty stressful in swarm and honey flow season, with all of the inspections and harvesting plus a (different) full time job. Would have been fine if I was retired, but I wasn’t.