if I was very poor in physics then how many marks can I get in jee main average plz tell me

You can't predict your marks based on one subject.
If you want to know where you stand among other competitors you should solve complete mock test series.

If you want to score good marks in JEE Main you have to work on your physics.

You can follow below points to crack this subject

1- You should refer past year problems to understand the level of questions asked in the exam.
2- Refer NCERT books to clear your concepts and to build strong fundamentals.
3- Solve MCQs related to important topics to understand the topics.
4- Focus on the topics which carries more weightage in the exam.

All the Best!!