But walder Frey and his c. 2000 men were not there at the trident. So not 20k. Also the north probably wouldn’t have had time to fully mobilize their armies.

Even still. Let’s say Walder Frey and his 4,000 men weren’t there, that’s still 16k from the Reach. Let’s say the North didn’t fully mobilize, that’s 18k. Let’s say half the Vale had to be left behind to hold it down due to loyalists, that’s 15k. That’s 49k total and that’s even before we get to Robert’s Stormlanders.