In total you could give 3 attempts. 

If you give both exams in Jan & April it would be considered a single attempt. Better of the two scores would be taken. 

And if you give either of attempts, it shall still be considered single attempt. That is exams in a given year shall be considered single attempt for evaluation.

I hope it clarifies you.

Chances very good and will solely depend on how you prepare and how good you score in mock exams. Its recommended to appear for 20mocks atkeast and take an average score of all to determine your minimum score possible in January 2019 attempt.

If your average is abive 100 marks you will have good chances to qualify for Jee Advanced 2019.

In 2019 students can appear for Jeemain twice in January and April so make best us eof both attempts.