If we are to take the Federation for American Immigration Reform's estimate (presumably where this figure came from) at face value, this would be true. However, it is important to note that FAIR was founded by a white nationalist and is classified by the Southern Poverty Law Center as a white supremacist hate group. Multiple leaders of FAIR have made statements about immigrants that most would consider racist and in favor of eugenics, the practice of limiting reproduction by "undesireables." This is important to consider in evaluating a potential motive/bias in what information they publish or studies they conduct.
According to the New York Times, illegal immigration into the U.S. has substantially increased under President Biden due to changes in policy, "global migration patterns" and migrant demographics. The Times provides the U.S. Customs and Border Protection's figure of illegal immigrant apprehensions since 2021, which is about about 5.92 million. Although this would indicate an estimate of illegal immigrants at likely over 6 million, this is still significantly lower than FAIR's estimate.
If we look at FAIR's explanation of why their estimate is much higher than the U.S. Customs and Border Protection's, there are a few key weak points. FAIR includes immigrants with protections such as refugee stautus or asylum seekers, like those from Ukraine, or minors brought over illegally. They also include immigrants given parole. These statuses all have certain legal protections in the U.S., so the discrepancy in numbers is due to differences in legal opinion.
Overall, estimating the number of undocumented, illegal immigrants is a difficult process, but I think it is safe to assume that this claim is overestimated, if not blatantly false or potentially racist. It is also important to note that FAIR makes claims as to why the U.S. government's estimates are false without providing evidence or references, like "For policy reasons, they now downplay or even entirely fail to account for undercounts and low response rates." FAIR makes this claim, but does not provide a source for it as it does for other claims in the article. Immigration is a controversial issue, so it's difficult to truly ascertain the number.