2 Questions on 72 hour free transit

Hello,
First: I will be arriving in Beijing in April 29, 2018 from the US with a valid passport. I will be staying less than 72 hours from arrival to departure. I fly out to Japan early on May 3, 2018. I believe this does not require a tourist travel L, correct?
Second: On my way home from Japan on May 11, 2018, I have a 17 hour layover in Beijing and depart for the US on the 12th. This would also be covered and not require an entry permit?
Both stays are less than 72 hours, however combined the stays are over 72 hours. Is there a limit of how many times I can use the 72 hrs free transit?
Thank you for your time in advance.