@Yair Ogen While installing the cluster, in the "Assign Slaves and Clients" page ambari will give an option to select the nodes where you want to install the Node managers. By default only a single node will be selected. You can select other nodes as well to install on them. The no of node managers you want to install depends on no of factors (no of applications you want to run, memory used by apps etc) Check this link for more info https://community.hortonworks.com/questions/82861/how-many-node-managers-should-be-installed.html
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