There is no limit on how many times you can use TWOV. As long as you enter and exit China from/to a different country (with HKG, Macao and RoC considered different for TWOV-purposes), TWOV rules will always apply, no matter how often you're using it (one could say, though, that if you've plenty of travel to China, it would make sense to get a multiple entry Visa, especially if your countries nationality gets long-term ones) - just make sure that the flights in&out are direct ones (no technical stops etc.) and you're fine with the rules that apply (ie, 72/144hrs etc.)