This may be true. While I couldn't find an entirely credible source stating 3.8 million migrants have entered the U.S. since Biden took office, I did find an article by the Pew Research Center stating "Southwest border encounters increased to their highest recorded level in fiscal 2021. The Border Patrol reported 1,659,206 encounters with migrants at the U.S.-Mexico border last fiscal year, narrowly exceeding the prior highs of 1,643,679 in 2000 and 1,615,844 in 1986." Of course, President Biden took office in January of 2021, and immediately rolled out executive orders allowing open borders. Ironically, President Biden campaigned against former President Trump's border wall in favor of an open border, however, he was recently called out for expanding the border wall in Texas as per an influx of migrants from the south. According to NPR “The Biden administration is moving forward with a controversial plan to build a new section of the border wall in Texas. It's waiving more than two dozen federal environmental laws to add 20 miles to the barrier. And that's even though President Biden previously pledged he would not add another foot of border wall. The White House made the decision while another surge of migrants comes across the southern border." In all, the exact number of 3.8 million may not entirely be true, there has been a great number of migrants coming across the border under the Biden administration as per his campaign promises and plan for open borders.