One scenario outlined by the United Nations Population Division, Replacement Migration (from year 2000) for the United States reads:

"In order to keep the potential support ratio constant at that level, it would be

necessary to have 593 million immigrants from 1995 to 2050, an average of 10.8 million per year. By 2050, out of a United States total population of 1.1 billion, 775 million, or 73 per cent, would be post-1995 immigrants or their descendants."

https://www.un.org/development/desa/pd/sites/www.un.org.development.desa.pd/files/unpd-egm_200010_un_2001_replacementmigration.pdf