According to Wikipedia, it's about 38%.
It depends on how you count them..........and what year you use.
Overall, using the official tourist arrival figures for 2019, the US sends approximately 50% of all tourist arrivals in the DR. The Banco Central breaks it down by counting US passport holders, non-resident Dominicans (majority likely from the US), and if you can believe it, a separate category for those from Puerto Rico (who are also US citizens).
The breakdowns are posted monthly/yearly at the website of the Banco Central;
Banco Central RD Nuestro objetivo principal es mantener la estabilidad de precios, por mandato constitucional y la Ley Monetaria y Financiera No.183-02 #BCRD