91e62454_munin_conf___Munin__Default_value
[Directive] max_graph_jobs [Explanation] Maximum number of parallel processes used by munin-graph when calling rrdgraph. Available since Munin 1.4.0 [Required] 6 [Default value] 
The optimal number is very hard to guess and depends on the number of cores of CPU, the I/O bandwidth available, if you have SCSI or (S)ATA disks and so on. You will need to experiment. Set on the command line with the -n n option. Set to 0 for no forking.
munin.conf – Munin
Default value