In Nintex Workflow 2007, this is a 5-digit number located at the bottom of the Licensing page at Central Administration > Application Management > Licensing.
Alternatively, each workflow action will show the build number in the lower-left corner of the action.
In Nintex Workflow 2010, this is a 4-digit number located at the top of the Licensing page at Central Administration > Nintex Workflow Management > Licensing.
Alternatively, this can be found within the Workflow Designer by selecting the Help drop-down menu from the toolbar menu, then selecting About Nintex Workflow.
All other Nintex build numbers can be found by accessing the corresponding product's Licensing page from Central Administration.