I had a similar question many months back. We had more than enough Humans to keep our army strong. However the UNSC WAS actively recruiting. I don't know if that means they were lacking troops or not. In Sadie's story she mentions the UNSC is taking anyone they can get. Doesn't sound too good, but perhaps they just want a constant flow of people. However if they HAD conscripted massive amounts of humans they would more resources to build armour, ships, gun, etc. It would put a larger strain on resources. So more humans would have meant more supplies needed which would have meant more money needed.