Email Send Now (to 50 or Less) - More than 50?
Its a hidden setting and can be set in below ways

Add below line in civicrm.settings.php
$civicrm_setting['Mailing Preferences']['simple_mail_limit'] = '100';

Via drush or cv
cv api Setting.create simple_mail_limit=100
drush cvapi Setting.create simple_mail_limit=60

Mysql statement
UPDATE civicrm_setting
SET value = '100'
WHERE name = 'simple_mail_limit';

Via api explorer