"The number of all the men ... is one hundred and fifty-one thousand four hundred and fifty." or "The number of all the men camped under the standard of Reuben, according to their divisions, is 151,450"
"When traveling, the camp of Reuben will start walking second" or "When the Israelites leave, those tribes will leave next". This means that when the Israelite camp moves, the camp of Reuben will start walking out after the camp of Judah goes out.