Skverma,
What is the limit of Activiti-Engine?
I don't know what you mean by this, but maybe these performance test metrics will give some insight into what you're looking for.
How much process Instance created in one Activiti workflow?
I do not believe there is any set limit to the number of processes; the limiting factor will be the size of your Activiti database.
How can I customize ProcessInstanceId?
There is java code within the Activit-Engine that generates the processInstanceId, and since Activiti is open source, you have the ability to modify whatever you'd like; however, I don't really think anyone would recommend changing this ID generation.
-JEarles
