Matched by need
Two sessions a day will do most of the work. What separates camps for this goal is whether food is handled on site, whether there is any structure beyond turning up, and whether the sessions are survivable twice a day from an untrained base.
A camp that promises a specific weight loss figure is selling you something. Nobody can promise that without knowing you.
4 camps
Chalong, Phuket / Large camp
The largest and best-known training complex in Thailand, running Muay Thai alongside a full MMA and BJJ programme. Big enough that you can always find training partners at your level, and structured enough that a total beginner is never left guessing. The trade-off is scale: it feels like a fitness resort rather than a Thai gym, and it is priced accordingly.
Chaweng, Koh Samui / Mid-size camp
The main established training option on Koh Samui, running a mixed programme with fitness classes alongside Muay Thai. Works well for a training-and-holiday split, or for someone travelling with a partner who is not training.
Pattaya, Pattaya / Large camp
Large resort-style training centre attached to the best-known equipment brand in the sport, with on-site accommodation and food. Polished and self-contained, which is either the appeal or the drawback depending on what you came for.
Pattaya, Pattaya / Mid-size camp
Runs structured multi-week training and weight-loss camps as packaged programmes rather than open drop-in training. Suits people who want the schedule decided for them.