const regex = new RegExp(totalUsers, 'g'); // g é para multiplas ocurrências
formula = formula.replace(regex, "USERS");

var str = "I have a cat, a dog, and a goat.";
var mapObj = {
   cat:"dog",
   dog:"goat",
   goat:"cat"
};
str = str.replace(/cat|dog|goat/gi, function(matched){
  return mapObj[matched];
});