Does Aws have the largest number of datacenter in the world?

es, AWS has one of the largest global data center footprints,

Does Aws have the largest number of datacenter in the world?

if the largest global cloud presence ,AWS is the leading in terms of regions and AZ. if the question is about most physical data center sites , Microsoft Azure is on the top

If a region in aws fail , how long will it take it to become live again?

if the service is configured on only one region ,recovery could take hours or days ,if the service is configured for multi.region failover it can be live again in seconds to minutes depending on the setup.

how the critical applications are maintained in aws ,if one availability zone is down ?

is that all the regions are connected ? ,if yes though which physical hardware its connected ?