If I am not happy with the hair transplant result, can I have second one immediately?

If there is an obvious asymmetry with the hairline, there is no reason to have to wait for a touch up procedure, it can be done anytime.

It is best to wait until the swelling goes down first.Then you should wait to see the hair follicles to grow back, roughly after 3 months.Also you should allow some time for the donor area to heal and then you can have a top up.
Good luck

During the transplant it is likely that the hairline was correct.  Swelling in the post operative period can make the hairline uneven for as long as two weeks.  Ideally, you would allow all of the swelling to abate before judging the symmetry of your hairline.  At that time, if things were uneven, you could plan for a future revision, if necessary.