Well that depends mostly on personal preference and occasionally on the thread. Lots of people will post all three or four of their cards once they are done. Some people don't post cards at all. If I'm in a swap I usually will post 2 cards then the next 2,. Though if I only do 3 cards then I post them all. And then some people do a card at a time.
If there is a thread that wants less posting, it will usually say in the description. But this is usually not about posting cards.