How many one can use a Visa-Free Transit in a year

Hello,
I was looking into flights that will be have layovers in Xiamen from the US before continuing to Singapore. However, the better round-trip ticket is US-Xiamen-Singapore-Xiamen-US. Most of the options are under 24 hours (one was 26 hours). Since I would be transit through China twice on a single trip, what are the rules on how many times one can use the visa-free transit through China en-route to a third country in calendar year?