Kinda like McClellan in the civil war stating the army of the potomic was 1/2 to 1/3 the rebel armies he faced. So basically a short hand way of saying our forces are greatly diminished.
Even if they meant 50 percent literally, they never give very solid numbers so what was the original armies size? The portrayed army in winterfell didn't look that enormous.
I'm also highly disappointed with Dany's general tactics in battle. She is a poor general.