Reworking my assumptions with the more accurate data thanks to @Malevolo
Total area is still about the same.
So 30 million hectare makes 153 million tons of wheat assuming low quality TP (20°C difference is hard to do at this tech level).
1 ton of wheat has 1 million calories means 153 million people theoretically possible. Subtract tens of millions due to low level of plant cultivation and diseases we can't really fight. But 100 million is still a solid estimate as to what is theoretically possible once we get the seed drill. Otherwise ~17 million on the upper end.
But once more, disease will murder us long before we get anywhere close to that level unless we sprawl out a lot more and have the lowlands export like crazy.
Before that, though, the Canal should allow us to boost the lowlands output considerably when they get full access to the Ironworks.