in one year its 2. However I quote the NTA FAQs-

As per the new scheme, the JEE (Main) - 2019 exam would be conducted twice in a year. The Number of attempts for admission in a particular academic session will be treated as One. The age eligibility as prescribed under clause Point No2.9 “Eligibility for appearing in JEE (Main) – 2019” allows a candidate to appear in JEE (Main) for three consecutive years.

Therefore one academic session is one year which has 2 attempts. Both the attempts will be taken as one attempt as the better of the scores will be used. so in total 6 attempts in 3 years if this is the first time you are appearing with 2 attempts per year.