<div class="col-6">

  <div class="mt-4">

    <select id="select-options" name="options[]" class="select is-invalid" multiple data-mdb-filter="true">

      <option value="1" data-mdb-secondary-text="Options 1 subtitle">Options 1</option>

      <option value="2" data-mdb-secondary-text="Options 2 subtitle">Options 2</option>

      <option value="3" data-mdb-secondary-text="Options 3 subtitle">Options 3</option>

      <option value="4" data-mdb-secondary-text="Options 4 subtitle">Options 4</option>

      <option value="5" data-mdb-secondary-text="Options 5 subtitle">Options 5</option>

      <option value="6" data-mdb-secondary-text="Options 6 subtitle">Options 6</option>

    </select>

    <label class="form-label select-label">Options</label>

    <div class="invalid-feedback">Cannot select 2 and 3 options</div>

  </div>

</div>