The Walmart computer in Bentonville tells the manager how many registers to open based on the volume, both historically and that day. They have a little discretion but if their discretion loses Walmart money, he won't be a manager very long. Bentonville can see every register, what is being rung up and how fast. Like most business these days, the machines are setting the pace for the humans. They are "chasing Old Sparky". (A dog racing thing for you northerners)