See Thai's website here and here for details on their luggage allowance. In summary, if you're flying economy, you get 20 kg. or if you're flying business class you get 30 kg. If you have a ROP Silver card, you get an additional 10 kg. and if you have a ROP gold card, you get an additional 20 kg. I believe if you have a Star Alliance Silver or Gold card from another airline, you also get those same additional allowances, but not sure if that's 100% correct with respect to the amount or for *A Silver members. Additionally, if you book online via the Thai Air website, you get another 10 kg.

So with a gold card, in business class, and booking online, you should get a total of 60 kg.

I think THAI is a bit more lenient of going over than the LLCs such as Nok Air or Air Asia, which I'd avoid if at all possible as they only give you 15 kg. I believe and so you'll end up paying much more in overweight charges. Not to mention THAI usually has more flights and is more likely to be on-time or close to it.