Hello SREELETHA

For JEE info please refer to this article

https://engineering.careers360.com/articles/jee-main-number-of-attempts

For BITSAT

Only Students who are appearing for 12th class examination in 2019 or who have passed 12th class Examination in 2018 are eligible to appear in the BITSAT-2019 test.

For GATE
There is no  age limit or you can give  as many as number of attempts in  GATE

but  t here may be an age limit for PG courses or PSU job after GATE So you have to think about it.

For UCEED

A candidate can attempt UCEED for a maximum of two times and that too in consecutive years.

for more please refer the official website of UCEED

http://www.uceed.iitb.ac.in/eligibility.html

I hope you get my point

thank you and GOOD LUCK!!!