According to [Fentanyl seizures at US-Mexico border up 750 percent][1]:


>The amount of fentanyl seized by border agents has skyrocketed in recent years, from two pounds in fiscal year 2013 to approximately 1,485 pounds from Oct. 1, 2016, to Sept. 30, 2017  

Matthew Barden of the Drug Enforcement Administration is [cited][2] as saying: 

>300 pounds of fentanyl is lethal enough to kill about 100 million people


So enough fentanyl was seized by border agents in FY2017 to kill every American. Plus, obviously not all the fentanyl is interdicted.  As long as 2018 amounts are at least as much as FY2017, the claim is defensible.  

Also, the word "fentanyl" is casually used to include derivatives thereof and [according to the American Council on Science and Health][3]: 

>One ounce of [Ohmefentanyl] (28 grams, 28,000,000 micrograms) would be enough to kill 175 million people—half the population of the United States [emphasis removed]


  [1]: https://www.washingtonexaminer.com/news/fentanyl-seizures-at-us-mexico-border-up-750-percent
  [2]: https://www.omaha.com/news/crime/pounds-of-fentanyl-among-drug-bust-records-this-year-for/article_37809613-706c-54a3-80c6-7c5a3c2b18a3.html
  [3]: https://www.acsh.org/news/2017/01/17/if-you-think-fentanyl-bad-10663