Hard to say. It's in the middle of the week, so that's usually less busy.

Are you before already in Sofia? If yes, you could buy the reservation there.

if it is fully booked, you could still take one of the many buses. But I read they increased capacity this year, so should work during week I think 
 

Unfortunately not. I’m coming into Turkey via Greece, then going to the Armenian border, then to Istanbul. Good to know I have the buses as backup though!

I See. If you need buses you can find them via obilet website. They show and sell tickets for most buses 

and out of curiosity: which route do you take from Greece to turkey? 

I’m flying from Thessaloniki to Izmir, because there’s no train, bus or taxi crossing that border You can also take the boat to Chios (Direct Ferries, gives a discount), then a boat to Cesme (turyolonline.com had to search my a** off to find this one) and a bus to Izmir (cesmeseyahat.net). Flying or boating costs nearly the same but differs an hour or 20. 

Oh i See actually there are buses from Saloniki, but they go only to Istanbul (you can find them also via obilet).

by ferry is indeed a long way, but should be beautiful. Have been to Izmir and Cesme last two years, but didn’t have the time to go via Greek islands but made it back overland by train.

enjoy your trip