In MR1, the mapred.tasktracker.map.tasks.maximum and mapred.tasktracker.reduce.tasks.maximum properties dictated how many map and reduce slots each TaskTracker had.
These properties no longer exist in YARN. Instead, YARN uses yarn.nodemanager.resource.memory-mb and yarn.nodemanager.resource.cpu-vcores, which control the amount of memory and CPU on each node, both available to both maps and reduces
YARN has no TaskTrackers, but just generic NodeManagers. Hence, there's no more Map slots and Reduce slots separation. Everything depends on the amount of memory in use/demanded
Using the web UI you can get lot of monitoring/admin kind of info:
NameNode - http://:50070/
Resource Manager - http://:8088/