What's the maximum population that Bulgaria could support?

Almost all European nations can support populations significantly larger than the ones they have. Bulgaria with double or triple its density at 15 - 20 million people could be supported easily - that would make it as densely populated as Poland (if double) or three-quarters as dense as Germany (if triple).
 
How much food can be grown an acre multiplied by how much arable land or how much food can they import to figure out a literal total, or asking how large could Bulgaria’s population grow with higher fertility rates?
 
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