My workflow has 50 constant data values rather create these variables for each instance, Is there a way to define them as constants at process definition level or so? So those will be used by process instances under that process definition.
Could you give more info on where you want to access the constants from? If you are utilising them from inside a service task then you could put them in your Java class. You can also add custom beans to resolve values in your BPMN at runtime from expressions.
Note Activiti Enterprise has Data Models that allow you to store constants inside a related DB table, but you will not have that if you are using the Community engine. See this blog post for more info on this feature.