It’s possible to create activation keys with limitations and even hardware-locked keys. So yes, you can offer a limited set of activation keys to your customers (1 per computer).
Deactivation is not supported yet. Maybe in the future, like for HTML Executable.
For 3), do you mean the code for your protected workbook or XLS Padlock itself?

Dear sir,

(1) Is there any limitation on each activation code. But can this program install at different PC? Any limitation on the numbers of PC I can install?
(2) If there is limitation … saying maximum 3 times of activation, can I deactivate one of it and install in another PC?
(3) I received a message saying that my existing activation code is no longer valid, why?