namespace app\models;

use yii\base\Model;
use yii\web\UploadedFile;

/**
 * UploadForm is the model behind the upload form.
 */
class UploadForm extends Model
{
  /**
   * @var UploadedFile file attribute
   */
  public $file;

  /**
   * @return array the validation rules.
   */
  public function rules()
  {
    return [
      [['file'], 'file'],
    ];
  }
}

<?php
use yii\widgets\ActiveForm;
?>

<?php $form = ActiveForm::begin(['options' => ['enctype' => 'multipart/form-data']]) ?>

<?= $form->field($model, 'file')->fileInput() ?>

<button>Submit</button>

<?php ActiveForm::end() ?>

namespace app\controllers;

use Yii;
use yii\web\Controller;
use app\models\UploadForm;
use yii\web\UploadedFile;

class SiteController extends Controller
{
  public function actionUpload()
  {
    $model = new UploadForm();

    if (Yii::$app->request->isPost) {
      $model->file = UploadedFile::getInstance($model, 'file');

      if ($model->file && $model->validate()) {        
        $model->file->saveAs('uploads/' . $model->file->baseName . '.' . $model->file->extension);
      }
    }

    return $this->render('upload', ['model' => $model]);
  }
}

public function rules()
{
  return [
    [['file'], 'file', 'skipOnEmpty' => false],
  ];
}

public function rules()
{
  return [
    [['file'], 'file', 'extensions' => 'jpg, png', 'mimeTypes' => 'image/jpeg, image/png',],
  ];
}

class UploadForm extends Model
{
  /**
   * @var UploadedFile|Null file attribute
   */
  public $file;

  /**
   * @return array the validation rules.
   */
  public function rules()
  {
    return [
      [['file'], 'file', 'maxFiles' => 10], // <--- here!
    ];
  }
}

<?php
use yii\widgets\ActiveForm;

$form = ActiveForm::begin(['options' => ['enctype' => 'multipart/form-data']]);
?>

<?= $form->field($model, 'file[]')->fileInput(['multiple' => true]) ?>

  <button>Submit</button>

<?php ActiveForm::end(); ?>

$form->field($model, 'file[]')->fileInput(['multiple' => true])

namespace app\controllers;

use Yii;
use yii\web\Controller;
use app\models\UploadForm;
use yii\web\UploadedFile;

class SiteController extends Controller
{
  public function actionUpload()
  {
    $model = new UploadForm();

    if (Yii::$app->request->isPost) {
      $model->file = UploadedFile::getInstances($model, 'file');

      if ($model->file && $model->validate()) {
        foreach ($model->file as $file) {
          $file->saveAs('uploads/' . $file->baseName . '.' . $file->extension);
        }
      }
    }

    return $this->render('upload', ['model' => $model]);
  }
}