You are mocking those people when you use this joke. Those you are speaking to may think it is a joke - not necessarily the 'so and so's' being referred to in the joke. The joke comes from only one person being needed to change a light bulb. Asking this question is a joke at the expense of those "so and so's". It's really calling them stupid as you're saying it takes several of them to figure out how to change the bulb.

Are you actually asking about a specific joke that uses the words "so and so's," or are you talking about "How many [X] does it take to change a lightbulb?" in general? (For example, "How many firemen does it take to change a lightbulb?" "How many lawyers does it take to change a lightbulb?" and so on.) If you're asking about "lightbulb" jokes in general, there's no single answer to explain every lightbulb joke. Each joke has a different punchline. You have to know the punchline in order to understand the joke. The "how many..." part is just the beginning of the joke, not the whole thing. The second part of the joke (the answer or punchline) is often a play on words. If you're asking about a specific joke that uses the phrase "so and so's," then we would have to hear the punchline to understand it.