Suppose you have been teaching in Thailand for two years as of now. It's time to apply for your work permit and visa renewal, but your school is now asking you that from here on, the 7-module is also mandatory. As I understand it, we are given three times (2 years * 3 = 6 years) to obtain a temporary license. After that, we only have to do 7 modules.  Your help will be much appreciated. 

How it generally works with teaching license waivers, is that you're meant to show that you are making progress towards becoming a qualified teacher.

So you should do at least 1x module during your 2 year license.

If you don't, they'll probably (not 100%) give you another license waiver anyway, maybe even 2, but they might include a more strict condition that next time they want to see that you've actually made some kind of commitment to register for the modules (e.g. Signed up for one, or attempted one, even if you haven't actually completed it).