Does CiviMail have an upper limit for total recipients in an email blast?
Absolutely - that can be done - the 50 is just a reference to the Search -> Find Contact screen -> Search -> Select -> Actions -> EMail send now (to 50 or less) option.

When sending out lots of CiviMail to lots of CiviContacts best to use an external SMTP service to help you with the Email delivery - and to minimize the risk that your webserver will be blacklisted. 

UPDATE an actual number from one of my clients' civicrm tables -> intended recipients: 89419 And I'm sure there were larger mailings.