How to limit number of characters in TextFormField?

use maxLength

TextFormField(
  controller: _deskripsiController,
  onFieldSubmitted: (String value){
        setState(() {
          _deskripsiController.text = value;
        });
  },
  decoration: const InputDecoration(
        border: const UnderlineInputBorder(),
        labelText: "Deskripsi",
  ),
  maxLength: 8,
)

Use inputFormatters property. The following code would limit the textFormField to 42 in length :

new TextFormField(
  inputFormatters: [
    new LengthLimitingTextInputFormatter(42),
  ],
);

UPDATE: Import the package first. import 'package:flutter/services.dart';