How many 72 hr transit visas can I apply for in one month?

I fly into Beijing, take advantage of the 72 hour free transit and then fly on to Singapore (within 72 hrs). From Singapore, I would fly back to Beijing, and the following day fly, out. Is it possible to get two 72 hour free transit within 2 weeks of each other? What are the limits on how many times you can apply for this?