Limit Image field to maximum 1 file for a template in ProcessWire

limitImageFieldMaxFiles.php

<?php namespace ProcessWire;

// put this in /site/ready.php

wire()->addHookAfter('ProcessPageEdit::buildForm', function (HookEvent $e) {
    /** @var ProcessPageEdit $edit */
    $templates = ['post', 'basic'];

    $edit = $e->object;
    $page = $edit->getPage();

    if (!in_array($page->template->name, $templates)) return;

    /** @var InputfieldForm $form */
    $form = $e->return;

    /** @var InputfieldImage $imageField */
    $imageField = $form->children->findOne('name=images');
    if (!$imageField) return;

    $imageField->maxFiles = 1;
});