Saw the following headline in Forbes:

"Fox News Nearly Sweeps Top 100 Highest-Rated Cable Telecasts In August."

https://www.forbes.com/sites/markjoyella/2021/08/31/fox-news-nearly-sweeps-top-100-highest-rated-telecasts-in-august/?sh=561107c6b746

According to the article, Tucker Carlson got an average of 3.3 million viewers.

Among viewers 25-54, the key demographic group for advertisers, Tucker Carlson Tonight finished first with an average audience of 551,000

So I looked at what his numbers were back in October of 2020.

Overall viewership:

Carlson - 6.315 million

Among viewers 25-54

Carlson - 1.245 million

Edit: Someone astutely pointed out that ratings are typically higher before a presidential election.

Here's some numbers from April of 2020 that should serve as a better indicator:

https://americandigest.com/tucker-carlson-passes-sean-hannity-on-fox-news-ratings/

From when Carlson eclipsed Hannity as their top draw:

Nielsen Media Research showed Hannity with an average of 4.4 million viewers and 809,000 in the all-important 25-54 age demographic.

On the other hand, Tucker Carlson averaged 4.6 million viewers and 885,000 in the 25-54 demographic. April was Tucker Carlson’s highest-rated month in both categories.

Still a significant decrease.