A minimal form sample with adjustable number of fields.

variable_field_number_form.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Form Demo',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final GlobalKey<FormState> _formKey = GlobalKey<FormState>();
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text('Flutter Form Demo'),
        ),
        body: SafeArea(
            child: Form(
                key: _formKey,
                autovalidate: true,
                child: SingleChildScrollView(
                    child: Column(
                  children: <Widget>[
                    ...List.generate(
                      5,
                      (i) => TextFormField(
                        decoration: InputDecoration(
                          icon: const Icon(Icons.calendar_today),
                          hintText: 'Please enter something to Field $i',
                          labelText: 'Field $i',
                        ),
                      ),
                    ),
                    Container(
                        constraints: BoxConstraints.expand(height: 55),
                        margin: const EdgeInsets.all(40.0),
                        child: RaisedButton(
                          child: const Text('Submit'),
                          onPressed: null,
                        ))
                  ],
                )))));
  }
}