Can you find the missing piece of the Porsche?

Can you replace the question mark with the correct number in the below table sequence?

Christina has four daughters, and each of her daughters has a brother. How many children does Christina have?

Can you find the mistake in the below statement?

When I get multiplied by any number, the sum of the figures in the product is always me. What am I?

Outside a massage parlour, there is a board that reads
"I only massage those who do not massage themselves."

Reading the above statement, does the masseur massage himself?

Solve the following series:
AZ, GT, MN, __, YB

If there are fifteen crows on a fence and the farmer shoots a third of them, how many crows are left?

One day, I thought of ways that can be used for creating a palindrome. So I decided that I will turn into a larger number by adding the reversed digits to the original number and keep doing it till I finally obtained a palindrome.

I am not sure if this process will always result in a palindrome eventually but I was able to produce a four-digit palindrome. Can you guess my starting number?

I have cities, but no houses. I have mountains, but no trees. I have water, but no fish. What am I?