The total number of men was 603550. ; Numbers

Numbers 1:01-54
The Lord spoke to Moses in the tent of meeting in the Desert of Sinai on the first day of the second month of the second year after the Israelites came out of Egypt. He said: Take a census of the whole Israelite community by their clans and families, listing every man by name, one by one.
Moses and Aaron took these men whose names had been specified, and they called the whole community together on the first day of the second month. The people registered their ancestry by their clans and families, and the men twenty years old or more were listed by name, one by one,  as the Lord commanded Moses.
From the descendants of Reuben was 46500
From the descendants of Simeon was 59300
From the descendants of Gad was 45650.
From the descendants of Judah was74600.
From the descendants of Issachar was 54400
From the descendants of Zebulun was57400
From the sons of Joseph was 40500
From the descendants of Manasseh was 32200
From the descendants of Benjamin was 35400
From the descendants of Dan was 62700
From the descendants of Asher was 41500
From the descendants of Naphtali was 53400
The total number was 603550. 
The ancestral tribe of the Levites, however, was not counted along with the others. The Levites, however, are to set up their tents around the tabernacle of the covenant law so that my wrath will not fall on the Israelite community. The Levites are to be responsible for the care of the tabernacle of the covenant law.
Numbers covers 38 years in the history of Israel: the period of desert wandering in the Sinai peninsula. It begins two years after the escape from Egypt. It ends on the eve of entry into Canaan. The title comes from the  'numbering' of Israel in the early chapters and chapter 26/
this book might have been called "The grumblings of  a nation". The trust in God which took them out of Egypt evaporated as they began to experience the hardship of desert life. Numbers in one long sad story of complaining and discontent. But God remains constant, present always with is people; and the hard lessons learned will enable Joshua to lead the new generation home.
Why?
The Lord already the behavior of Israelite what will be, how they will complain in their life. So God asked Moses make census and let the record to know how the Israelites become and what tribe will be the leader in Israelites.
The Levites was separated as God's servant, they are to be responsible for the care of the tabernacle of the covenant law.
There were total 603350 men, if including women and children, there were over million people group, the living cost were very high when they were in the desert. But God is the provider to his people.
How?
Trust in the Lord, do not complain but thanks to the Lord for God loves us even sometime God let us be trained to growing up may bear more responsibility.