Ok, then what is your estimate on the numbers each side brought into play on Stepstones? Also regarding your point about the Disputed Lands being a potential recruiting pool for the Nine, doesn’t AWoIaF state its pretty desolate nowadays( btw I agree with you that this fact doesn’t make much sense) ?

Well, I don’t agree with the Disputed Lands being desolate – classical or Renaissance city-states cannot survive without a rural hinterland to feed them, because you can’t feed a population of that size solely through imports with the limited transportation technologies available at the time. I’m sure the Disputed Lands get ravaged by armies all the time, but so was rural Italy during the wars of the Renaissance and they were still producing crops (indeed, a big part of the reason they were being ravaged is that they had crops to feed armies with).

I would guess ~40,000-50,000 on each side, making it a significant war by the standards of the First Blackfyre Rebellion or Robert’s Rebellion.