First of all you should only increase the number of work processes if you are sure there are sufficient CPU resources to cope with the increase. Normally there should be at least one or two Dialog work process in SM50 without any CPU time. Monitor your CPU utilization when increasing the number of work processes.
But also be concerned if the memory usage is over 90% during peak dialog hours as this would indicate that your hardware resources are not sufficient for the workload.
See also SAP Note 39412 'How many work processes should be configured?'