You can set the numbers in the webbased configuration. However,this affect all workflows.
AFAIK there is no way to delete the token e.g. in a script or another workflow.
Maybe you can delete them manually in the database (but thats not supported, I guess). For that, find some ideas here:
In the web configuration / General / advanced configuration the "Max number of runs" will make sure there will not be more then this number of runs per workflow.
Removing of completed w/f run's is done automatically. Starting from vCO 5.5.2 it is possible to configure the specific time/period when the clean job is run using cron expression
For example to configure custom schedule for the clean job set following property in vmo.properties file:
By default clean job is run every hour
com.vmware.o11n.purge.executions.cron=0 0 * * * ?
Details for cron expression can be found here.
As already explained number of preserver w/f runs is configurable trough vCO Configurator.
Check in your vCO/vRO Client under: Workflows\Library\Configuration\Troubleshooting
Right-Click on "Delete all workflow runs", then select "Schedule Workflow"