For NTSE stage - 1 2020 Examination, the NCERT does not release any official syllabus for the exam. The question paper is divided into 2 sections named MAT ( Mental Ability Test ) and SAT ( Scholastic Aptitude Test ).
MAT includes questions for testing the mental ability.
SAT includes questions from Science, Mathematics, Social science of Class 9th and 10th.
Refer here for more info - https://school.careers360.com/exams/ntse
Feel free to ask doubts in the Comment Section.
I hope this information helps you.