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.