With regard to contributions made for years prior to 2020, you are asking about converting from a traditional IRA to Roth, not recharacterizing. Recharacterizing is something quite different; as of this moment you can potentially recharacterize only contributions made for 2020 (or for 2021 if you've already made an IRA contribution for 2021), but only if you are eligible to have made the Roth IRA contribution because your modified AGI for the year for which the contribution is made is not too high.
There is no limit on the amount that you are permitted to convert to Roth. How much you choose to convert in a given year is just a matter of how much taxable income you are willing to incur in a given year from converting. If you have made nondeductible traditional IRA contributions, your conversions will be at least partially nontaxable, but you'll need to run through a simulated tax return to determine that. For example, if you want to estimate your increase in tax liability from converting some amount in 2021, you'll need to simulate your 2021 tax return with and without a particular amount of Roth conversion.
