The number of MapReduce jobs that you want to process the input file depends on the split size. The longer the block size, the longer run time for a single job. If you split the input file into multiple parts based on the split size, a separate job processes each part, which improves the run time.
For example, the input file size is 112 MB. If the block size is 128 MB, the HDFS stores the input file in a single block, and a single job processes the file. If you set the split size as 32 MB, the input file is split into 4 parts and 4 jobs process the file.