It depends on the ones own ability to handle different chilli peppers. In general the small hot rocket variety of thai chilli pepper is used in many different dishes.
Let start with a basic "Som Tam" or Papaya Salad. This dish while fresh and uncooked has blown the roof of many foreigners mouth when first tasting thai food. If you are not used to the hot spiciness of these peppers, they have a different kind of hot then a Jalapeno pepper. Faster burn intense and on fire. If you a beginner in Thai food but willing to feel some warmth i suggest you try one chilli pepper in the dish about 1-2 helping size. If you want to really feel a burn try 3 and see if you are able to not hiccup or run for the water bottle. And if you are fully Thai ready to handle all the burn i recomend putting 5 chilli's in your dish. It works to taste. So remember taste it often. And if you are new to Thai food and you dont seem to have the flavor right you may be lacking one more key ingredient. Fish Sauce. Similar to Soy Sauce in saltiness but that is it. THis is added to most thai dishes, even desserts. One or two splashes is all you need for the beginner. If you are Thai you would be served a small dish full of fish sauce and diced up chills to put on top of you dinner. If it wasn't hot enough this will make you run for a napkin to towel of your forehead. Thai food is very good and fresh. I recomend anyone to try and make it yourself. You can find the ingredients in you local grocer or maybe your back yard herb garden. Thank you and please come and see my interesting stone specimens we have. Let me know if you find this helpful?