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  there 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!!!