Since 2006 I have been working on the Homeless Shelter Directory. During this time I have tried to find good statistics on the number of homeless in the US.
In my research I have found that the National Alliance to End Homeless reported in Jan 2009 that due to the recession, they are estimating that there will be an additional 1.5 million Americans homeless over the next two years. They have their calculation in this report, Homeless Increase.
The National Coalition for the Homeless provides the standards for statistics for the homeless in the US. The breakdown the homeless population as Homeless Families with Children, Homeless Youth, Homeless Veterans and Elderly Homeless. There are many statistics in their reports and if you have time, I recommend you try to give it a good read, National Coalition For The Homeless Statistics.