Is there any limit to the number of 72 hour transit visas you can obtain in a month?

Hi! We would like to travel from Busan, South Korea directly to Shanghai, stay in Shanghai for two days, and fly out to Phnom Penh Cambodia. Two weeks later we will fly from Bangkok, Thailand to Busan, South Korea, but we have a two hour layover in Shanghai. Would this situation cause any problems? Thanks so much for your help!