Travelers admitted to the United States under the Visa Waiver Program (VWP) may generally stay for up to 90 days for tourism or business. The 90-day limit applies to the individual VWP admission and is not a 90-days-in-180 rolling allowance. Admission and the authorized period of stay are determined by U.S. Customs and Border Protection (CBP), so leaving and re-entering the U.S. does not guarantee a new 90-day period.
https://travel.state.gov/content/travel/en/us-visas/tourism-visit/visa-waiver-program.html