Namespace spans across all nodes.
Node - is an aerospike process. It runs on a server - VM or physical host. In production, you will run one node on one server. (For playing around, it is possible to start multiple nodes on same server and form a cluster by separating the IP addresses.)
In a brand new cluster, you must shutdown all nodes. Edit the config file in each node. Namespace definition for config parameters identified unanimous and static in each .conf file have to be identical for the cluster to form- for example replication-factor. Other parameters such as memory size may be different but that is a seriously bad idea. (BTW, Aerospike does not check namespace memory size you specify against actual available RAM. Burden is on you to go through capacity planning upfront.) So for practical purpose, each node should have identical namespace stanza - except for perhaps device name differences. I am assuming you are running one aerospike node (process) per server. The cluster forms by each node pointing to one or more different nodes in the mesh-seed-address-port config parameters.
Once you have correctly setup all config files, start each node one by one. All nodes should have the same namespaces defined for a proper cluster to form.
Use the search tool on this forum to understand all the nuances. There are plenty of knowledge base articles and discussions on this topic. Search for “Adding a Namespace”. Aerospike does not allow adding or deleting namespaces on an active cluster. Entire cluster has to be shutdown and there are other subtleties related to preserving existing data - non-trivial to add a namespace in a running production cluster.
Since you are setting a brand new cluster, I would do:
/etc/init.d/aerospike stop (on all nodes)
edit config file on all nodes.
/etc/init.d/aerospike start (node by node)
However, plan upfront for what namespace(s) you will need before getting into production.