This is such an open ended question that nobody can give you an accurate answer as there are so many factors that need to be considered.

How big is the site? How much data is being stored? What is the DB backend? How does it handle failover between DB servers in the event the primary goes down? Is it being hosted in a cloud service, your own DC or a cupboard? Does the location it is being hosted already have redundant power and internet connectivity? Are they diverse, so if one provider fails the other one will remain online? You’d need to maintain separate sites in case one location goes down due to a major event like an earthquake, so you need to replicate data in real-time to your DR location.

There are so many factors to consider and I haven’t named them all. Regardless of what your answer is, it would be very expensive to maintain any server at 6 9’s level of availability. For a marketplace website with only 1000 visitors a day there is no need for that level of availability because there would be times in the day that nobody would be on it. Only a marketplace the size of Amazon would consider that level of availability.