The standard (federal) qualifying rule is - you must process over $20,000 AND over 200 gross transactions through Shopify Payments (does not include third-party payment providers) in a given tax year. Both of these thresholds must be met, not just one. These thresholds are set by the IRS and not Shopify. The IRS mentions these thresholds on their website here.
Please also note these are gross figures i.e. transactions amounts before any refunds, chargebacks, fees or adjustments are deducted.
Most states follow the standard IRS threshold. However, in the last few years, several state-level governments have passed their own legislation which now requires Shopify to file 1099-K forms for merchants at much lower thresholds.
For the 2020 tax year, these are the states which have enacted lower reporting thresholds
Your state will be determined by the address you provided for your Shopify Payments account, which is separate from (and may or may not be the same as) the store address in general settings.
If you qualify for a 1099-K form you will see the document appear in the Actions > Documents tab on their Shopify Payments payout page. It's important to note that you need to be logged in as the store owner in order to see this tab.
If you would like to replicate the exact calculation of the 1099-K, to confirm the amounts which would have been reported if you qualified, these are the steps you would follow:
Let me know if you have any questions.