How to change hard coded limit of 50 transactional emails?
It's recommended to use CiviMail for larger mailouts. It's worth understanding the potential consequences of using "simple CiviCRM mail" to do mails in bulk; eg:


Harming your own site's spam reputation by sending many emails without spam compliant unsubscribe links.
Harming CiviCRM platform email reputation by using CiviCRM without respect to spam compliance.
Timeouts and performance issues when trying to deliver large numbers of emails in a single request.
... ?


So, this has long been hard-coded in an attempt to avoid people changing it. The limit is no longer hard-coded; it's become a hidden setting: simple_mail_limit. You should be able to change this setting from the default either by using the CLI tool cv:

cv api setting.create simple_mail_limit=99


Or by adding a setting override in civicrm.settings.php:

global $civicrm_setting;
$civicrm_setting['other']['simple_mail_limit'] = 99;


See configuring hidden settings in CiviCRM for more info, and do take note of advice here to reflect on why you think you need to change this value!