```js
let maxRowsCount;

const mockRowsCountFetch = async () =>
  maxRowsCount = await mockServerCall(2000, () => 5);

// a form with 10 items, with each item holding a username and password fields.
const form = new Form({
  children: Array.from({length: 10}, () => ({
    children: [
      {
        name: 'username',
        validate: validation(
          validation.required(),
          validation.async((value) =>
            mockServerCall(1000, () => value === 'john' ? 'username is taken' : null)))
      },
      {
        name: 'password',
        validate: validation(
          validation.required(),
          validation.validate(
            value => value && value.length < 6 ? 'invalid password' : null,
            {events: ['blur']}))
      }
    ]
  })),

  validate: ({event}) => {
    if (event === 'init') {
      return mockRowsCountFetch().then(() => {
        const sizeError = validateFormSize();

        return sizeError
          ? form.hasErrors
            ? [
              ...form.errors,
              sizeError
            ]
            : sizeError
          : form.errors;
      });
    }

    if (maxRowsCount !== undefined) {
      return validateFormSize();
    }
  }
});

const validateFormSize = () => {
  if (form.children.length > maxRowsCount) {
    return `Maximum allowed number of rows is ${maxRowsCount}`;
  }
};

// call 1 to the server is initiated because of the init validation

const username = form.children[0].get('username');
const password = form.children[1].get('password');

console.log(username.hasErrors); // false
console.log(username.errors); // null
console.log(password.hasErrors); // false
console.log(password.errors); // null
console.log(form.hasErrors); // false
console.log(form.errors); // null

username.onChange('john'); // initiates call 2 to the server

console.log(username.hasErrors); // false
console.log(username.errors); // null
console.log(password.hasErrors); // false
console.log(password.errors); // null
console.log(form.hasErrors); // false
console.log(form.errors); // null

password.onChange('123');
password.onBlur();

console.log(username.hasErrors); // false
console.log(username.errors); // null
console.log(password.hasErrors); // true
console.log(password.errors); // ['invalid password']
console.log(form.hasErrors); // true
console.log(form.errors); // ['invalid password']

// ... waiting asynchronously
// later, after response 2 comes from the server, but before response 1 comes from the server

console.log(username.hasErrors); // true
console.log(username.errors); // ['username is taken']
console.log(password.hasErrors); // true
console.log(password.errors); // ['invalid password']
console.log(form.hasErrors); // true
console.log(form.errors); // ['username is taken', 'invalid password']

// ... waiting asynchronously
// later, after response 1 comes from the server

console.log(username.hasErrors); // true
console.log(username.errors); // ['username is taken']
console.log(password.hasErrors); // true
console.log(password.errors); // ['invalid password']
console.log(form.hasErrors); // true
console.log(form.errors); // ['username is taken', 'invalid password',
                          // 'Maximum allowed number of rows is 5']

const tailChildren = form.children.slice(-6);

tailChidren.forEach((row) => row.remove());

console.log(username.hasErrors); // true
console.log(username.errors); // ['username is taken']
console.log(password.hasErrors); // true
console.log(password.errors); // ['invalid password']
console.log(form.hasErrors); // true
console.log(form.errors); // ['username is taken', 'invalid password']

tailChidren.forEach((row) => form.addChild(row));

console.log(username.hasErrors); // true
console.log(username.errors); // ['username is taken']
console.log(password.hasErrors); // true
console.log(password.errors); // ['invalid password']
console.log(form.hasErrors); // true
console.log(form.errors); // ['username is taken', 'invalid password',
                          // 'Maximum allowed number of rows is 5']
```