About 25 million
Actually, the above number is incorrect. The number of "immigrants" depends completely on what one means by "immigrants" and the time-frame one looks at.
There are many different categories under the immigration law, most of which do not give the beneficiary any rights to remain in the United States permanently but only some form of temporary residency (e.g. temporary workers, visitors, students, etc.). Likewise, individuals obtaining "citizenship" would also be excluded from this number since an individual must have already immigrated to the United States to qualify for citizenship and, as such, they would not be considered a "new" immigrant.
According to the Department of Homeland Security, the total number of immigrants entering the United States as Legal Permanent Residents since 1820 is precisely 75,356,722
In 2009, only 1,130,818 new immigrants came to the United States.
As of 2009 a total of 12,450,000 Legal Permanent Residents were living in the United States and 743,715 Naturalized to become United States Citizens.
Source: DHS "Persons Obtaining Legal Permanent Resident Status: Fiscal Years 1820 to 2009"