Here are the stats of the current status of mobile phone penetration in Thailand from GSMA. As you can see 89% of users are prepaid. With a population of 70 million people there are 83 million connections. This means many users have two or even more lines they are using.
This basically means that everyone has at least one phone and many people have a secondary device either for Internet or iPad device. Sim Cards are also finding uses in tracking devices and ATM machines modem. Plus with 20 million tourist yearly, you can expect many of these are activating SIM cards.
We should expect these numbers to continue to go up over the next several years. Mobile data speeds are getting faster and maybe in a lot of cases replacing DSL or cable modem needs. The carriers have also introduced one number, multiple SIM card programs. This way five Sim Cards can be under one account making adding top of credit much easier.