possibly 50,000-ish, considering not all Mandos were Neo Crusaders
After the recruitment of conquered folks in the second year of the war, their forces grew four fold. And all conquered folks were made into Neo-Crusaders. By that time, they had attacked the Republic. Not very likely they were only 50.000 in that time. They made up the majority of Mandalorian forces during the Mandalorian War, and they had become formidable enough to challenge the Republic.
Kandosii-type dreadnaughts carried a maximum of 40.000 troops including the crew. So it's well over that number.