The $5 trillion number is an estimate of what the maximum amount could be based on an interpretation of Romney's remarks by outside groups. His proposal isn't that specific.
But is he actually proposing $5 trillion in cuts over the length of his program?
Romney essentially wants to lower the tax rate and offset the loss in revenue by eliminating many tax deductions, credits, etc. Obama is correct in saying that eliminating deductions alone likely won't pay for all of it but Romney argues that the economic growth caused by the lower rates will make up that difference.