Before June 2018, a seller generally had to have a physical presence in a state before that state could require it to collect sales tax. South Dakota v. Wayfair, Inc. ended that rule. States can now require remote sellers to collect tax based on economic activity alone.
Since then, 45 states and DC have enacted economic nexus laws. The most common threshold is $100,000 in sales into the state. California and Texas use $500,000, and New York requires $500,000 plus 100 transactions.
The practical result for a growing brand is that your obligations can spread across many states without any change in how you operate. A store on Shopify that reaches $2M can easily have crossed thresholds in a dozen states.
The fix is to measure continuously rather than annually. Tracking trailing 12-month sales by state, and registering as you approach each threshold, is far cheaper than being found later.