https://en.wikipedia.org/wiki/Illegal_immigration_to_the_Uni...
About 11 million of those are illegal/undocumented immigrants. The remaining 34 million are people on visas, people with green cards ("lawful permanent residents"), and naturalized citizens of the United States. This does not include the children born to them, who are natural-born citizens, and are thus not counted as immigrants.
45 million is 14.3% of the US population, which might seem high, but if the list on Wikipedia is sorted by % of population, the U.S. is nowhere on the top of the list. Canada, Australia, New Zealand, Sweden, Norway, Switzerland, Ireland, Austria, Singapore, etc. all have more immigrants per capita than the United States. And gulf countries like the U.A.E. and Qatar have the highest.