Ryanair have advertised 250,000 passengers a year for Burgas.
We know they are planning 19 weekly flights during the summer schedule (though perhaps not all flights will operate for the full season) adding up to about 180,000 passengers between April and October.
This leaves 80,000 passengers for the winter schedule or about 7 to 8 weekly departures. Of course, a lot can change between now and when the winter schedule is announced.