There is no limit on the amount that you are permitted to convert to Roth. Whatever amount you convert in a particular year is reported on a Form 1099-R for that year that you receive from the traditional IRA custodian and that Form 1099-R must be reported on the tax return for the year in which the distribution from the traditional IRA occurred.
It's up to you to decide how much to convert based on how much additional tax liability you are willing to incur for that year.
There is not limit to the amount of a Traditional IRA you can convert to a Roth IRA. This is a taxable event, so the amount you convert will be subject to tax. If you do the conversion in 2021, it will be reported on your 2021 tax return that you will file in 2022. You will receive a Form 1099-R from your Traditional IRA custodian that you will enter to report the conversion.
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Even if I convert all to a Roth before April 15, 2021, do I have to claim it as Traditional in 2022? I thought I can contribute till April 15 and file tax till the date for this coming tax season....