I read somewhere that it is not possible to create namespaces dynamically. Does this still hold? If one creates empty namespaces, what is the memory overhead per namespace?
Yes, Aerospike at the moment does not support creating namespaces dynamically.
In the community edition, there is no fast start, hence no shared memory arena. So the overhead of a non-used namespace is minimal (30-40MB).
In enterprise edition with fast-restart, each namespace creates a 1GB shared memory arena, hence overhead of each namespace will be 1GB.