I've created a codesandbox which includes the following code, previously you were storing the value as a string, it'd be best to store it as number so you can use that to map out, which I do via the Array() constructor (this creates an array of a fixed length, in this case the size of the divCount state - when we update the state by changing the input value this creates a re-render and thats why the mapping is updated with the new value)

import "./styles.css";
import * as React from "react";

export default function App() {
  //state
  const [divCount, setDivCount] = React.useState(0);
  //function
  const appendChilddiv = (e) => {
    e.preventDefault();
    // Submit your form info etc.
    setDivCount(0);
  };

  return (
    <div>
      <form className="a" onSubmit={appendChilddiv}>
        <input
          value={divCount}
          onChange={(e) => setDivCount(Number(e.target.value))}
          type="number"
        />
        <button type="submit">submit</button>
      </form>

      <div>
        <form className="b">
          {Array(divCount)
            .fill(0)
            .map((x, idx) => (
              <div key={idx}>Div: {idx + 1}</div>
            ))}
        </form>
      </div>
    </div>
  );
}

You can do the following.

First thing it does is creates a new array of some length number.

Next, it fills that array with undefined, because creating new arrays like this doesn't really create an array of that length.

Lastly, we map over this array, we use the index as our key. We return an empty div for each item in the array.

Note, using an index as a key isn't the best idea. In general it should be something as unique as possible. If you have data you can use that is unique, then you should use that as a key.

return new Array(number).fill(undefined).map((_, key) => <div key={key}></div>);

You can map over an array whose length is same as that entered in input & create divs (if number entered is valid):

{!isNaN(number) &&
              parseInt(number, 10) > 0 &&
              Array(parseInt(number, 10))
                .fill(0)
                .map((_, idx) => <div key={idx}>Hey!</div>)
}

1. isNaN checks is number is valid
2. parseInt(number, 10) converts string into number,
3. Array(n) creates a new array with n elements (all empty) (try console logging Array(5)) - so you need to fill it
4. .fill(n) fill the array (make each element n)
5. and map is used to render different elements from existing things

So, in this way, you can achieve the mentioned result.

Here's a link to working Code Sandbox for your reference