Mass SMS recipients count
Kudos to Demerit and petednz-fuzion for helpful pointers, see below:

Demerit: Dummy SMS extension
petednz-fuzion: check your log_civicrm_option_value table for option_group_id = 35 to see who changed it


It is possible to find recipients of each mailing (even scheduled/not sent) via MySQL following these steps:

Table log_civicrm_mailing gives us our Mass SMS id
Table civicrm_mailing_recipients contains all recipients of all mailings (mass sms included) in the format:
mailing_id/contact_id/email_id/phone_id
From here we can explore each &quot;phone_id&quot;  record in civicrm_phone table, where we can see &quot;phone_type_id&quot;.

And this is a funny part: all my 5 recipients have phone_type_id=2 (&quot;Phone&quot;) while &quot;Mobile&quot; phone type has phone_type_id=1. So, it seems Mass SMS in my case checks if phone type is Phone, not Mobile.
Advanced Search by &quot;Phone type=Phone&quot; returns only 3 numbers, but it seems to be another story. Perhaps, it worth checking what exactly happens on line 227 of \CRM\Mailing\BAO\Mailing.php in this regards:
'location_filter' =&gt; CRM_Utils_SQL_Select::fragment()-&gt;where(&quot;$entityTable.phone_type_id = &quot; . CRM_Core_PseudoConstant::getKey('CRM_Core_DAO_Phone', 'phone_type_id', 'Mobile')),