This is sometimes called "Chinese" vegetable stew in Thailand, because the chosen vegetables are usually of Chinese origin. You can make substitutions, and some recipes for this stew call for as many as 10 different vegetables. You do need at least 4 to get the veriety of tastes and textures required, but if the produce market yields more on the day, then you can indeed use more (keep the overall quantity about the same.)
Though called a vegetable stew, this is by no means a vegetarian dish (For a vegetarian dish, see Kaeng Liang). It contains a substantial amount of meat. The essential flavor of the dish comes from the belly pork. If you prefer you can use a pound of belly pork and omit the poultry, but you shouldn't omit the pork altogether.
This is a one plate meal - a single dish forming the entire meal (as opposed to the more usual dining style in Thailand with several contrasting dishes).