Quentyn claims dorne is 50 thousand how could they double their forces by the time he reached daenerys

Quentyn is repeating a historical falsehood:

“If? The word is when. Dorne is the least populous of the Seven Kingdoms. It pleased the Young Dragon to make all our armies larger when he wrote that book of his, so as to make his conquest that much more glorious, and it has pleased us to water the seed he planted and let our foes think us more powerful than we are, but a princess ought to know the truth. Valor is a poor substitute for numbers. Dorne cannot hope to win a war against the Iron Throne, not alone. And yet that may well be what you have given us. Are you proud?” The prince did not allow her time to answer. “What am I to do with you, Arianne?”

Dorne is believed to have 50,000 spears, but only has 25,000 and prefers to let the rest of Westeros believe that as a cheap form of deterrance. 

Now, Dorne may have had more men in the past – for example, King Ferris Fowler led 10,000 men to war against Garth Goldenhand, while only controlling a small part of Dorne. My guess is that the brutal wars that Dorne experienced after the Targaryen invasion – involving as they did the mass bombardment of castles and holdfasts by dragons, scorched-earth tactics involving the destruction of wells and other sources of water, and total war directed against civilian populations – massively affected Dorne’s demographics similar to the impacts of the various Mongol invasions.