There may be situations in which you need to adjust the default configuration SharePoint 2010 uses to handle resource allocation for workflows, such as the maximum number of new instances. While there is not an out of the box GUI interface to throttle workflows, PowerShell is to the rescue with some straightforward options to both view and set workflow throttling via the SPFarmConfig command.

Bear in mind that the default configuration is in place to ensure solid performance and you should measure any changes against the power of your environment.

Here are the stated limits for workflows:
http://technet.microsoft.com/en-us/library/cc262787.aspx#Workflow

Here is the command for getting the farm settings:
http://technet.microsoft.com/en-us/library/ff607745.aspx

Here is the command for setting the farm settings:
http://technet.microsoft.com/en-us/library/ff607962.aspx