const x = "---x---x---x------x";
let input = 2;

let output = [];
for (const dashes of x.split("x")) {
  output.push(dashes);
  if (input > 0) {
    input--;
    output.push("o");
  } else {
    output.push("x");
  }
}
output.pop();
output = output.join("");

console.log({ output });

You can just loop over the and replace x with o until value becomes 0(which is falsy value)

let str = "---x---x---x------x";
let value = 2;

while (value--) {
  str = str.replace("x", "o");
}

console.log(str);

based on solution here:

var str = "---x---x---x------x"
var n = 0
var N = 2
var newStr = str.replace(/x/g,s => n++<N ? 'o' : s)