# gathered Judah together

"gathered the people of Judah together"

# registered them ... numbered them

He had someone write their names on an official list.

# by their ancestors' houses

"by their ancestors' families". See how you translated a similar phrase in 2 Chronicles 14:2.    

# all of Judah and Benjamin

"all the people of Judah and Benjamin" 

# twenty years old and upward

"20 years old and older"

# found them to be

"learned that there were"

# chosen men

"capable soldiers" or "skilled warriors"