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In the JoSAA (Joint Seat Allocation Authority) counseling process, candidates typically receive their seat allotment results across several rounds. The seat allotment for the first round is generally announced shortly after the choice-filling period closes, which includes mock allocations to help candidates gauge their chances.

As for your specific case with an ST preparatory rank of 550, your chances of getting a seat in a preparatory course will depend on various factors, including the number of seats available, the number of candidates with higher ranks, and the specific preferences you select during the choice-filling process.

It's essential to monitor the rounds of seat allotment closely. If you are allotted a seat in the first few rounds and choose to accept it, you can either freeze your seat or opt to float or slide, depending on your preferences for future rounds. You will know the outcome of your application in each round, and if you don't receive a seat in the rounds, there is a possibility of getting an offer in subsequent rounds based on the availability and demand.

Make sure to check the JoSAA website for the latest updates and details about your seat allotment status.