Dim sum traditionally comes in multiples of 3 or 4 in each steamer basket. However, there doesn't seem to be any resource to let the customer know whether you are getting 3 or 4 dumplings/items.
Is there a hard and fast traditional rule for each type of dim sum? Do har gow 蝦餃 and shu mai 燒賣 always come in 4s or do some restaurants serve them in 3s? What about wu gok 芋頭角 or char siu bao 叉燒包 - do they come in 2s or 3s or sometimes even 4s?
Factually, when dining at a dim sum restaurant, how do people know how many they will get? Is it always the same amount (a sort of tacit knowledge that regular customers pick up from experience) or, factually, can it differ between restaurants?