Hello,
You only get two consecutive attempts at JEE Advanced. First attempt is just after you pass in your class 12 and the second attempt is a year after you pass class 12.
For JEE Mains, you get three consecutive attempts in three years. In a year, you get two attempts at JEE Mains, one in January and in April.

So , If you have just passed your class 12, so you can take admission into Bsc course and prepare for JEE Mains and JEE Advanced as well.
But it would be really a hard process. You need to focus both on your Bsc studies as well as JEE preparation. You can also take a year drop and prepare for JEE Advanced.
So think wisely.
Please visit this link JEE Advanced preparation strategy

Please visit this link for JEE Advanced eligibility criteria
Hope it helps

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