Looking into the implementation I saw that calling the map-step with one scan results in exactly one mapper used. This is why the input set is not split at all.
Using a list of scans, giving it to the TableMapReduceUtil.initTableReducerJob
function, the input set is split at each scan. Thereby one can define the number of mappers used in the MapReduce job.
Another way would be to extend the TableInputFormat
class and rewrite the split method.
As Arnon Rotem-Gal-Oz said correctly, one can only access one row at a time within the mapper's map function.