Hey,

You have failed your exam so technically you are eligible for compartment exam and not improvement exam. The compartment dates are provided by the CBSE according to them and you will have to register for it to give your compartment exam.

As far as improvement is concerned, you can give improvement for as many subjects as you want without any restrictions. And no you won't have to give practical exams again. The practical marks of previous year is counted.

Hope that helps.

Thank you