how to display no. of character user can type and how many characters user had typed in TextFormField Flutter

You can add maxLength property (it will show the writtenCharacters/totalLimit)

TextFormField(
  maxLength: 45,
)

Or you can also have alternative (it will not show the writtenCharacters/totalLimit), but for this you need to import

import 'package:flutter/services.dart';

TextFormField(
  inputFormatters: [
    LengthLimitingTextInputFormatter(45),
  ],
)

You can still use a TextField and achieve what a TextFormField do by saving the input of the TextField to a variable on text changed

TextField(
          maxLines: 3,
          maxLength: 30,
          onChanged: (value) {
             _content = value;
            },
          ),

and send _content on your form submit

With a TextFormField you can have the number of input character by passing an TextEditingController to the TextFormField

final _myTextEditingController = TextEditingController();

TextFormField(
                controller: _myTextEditingController,
                maxLength: 30
)

and show the number of character with

_myTextEditingController.text.length