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

I am looking for a word that change both gender and number when i add the letter 'S'.

What Word am I Looking For ?

How many letters are in the alphabet?

John enters a small room. The Door Closes.
When the door opens, John is in a larger room.

Explain?

How many times can you subtract the number two from the number fifty?

Samuel was out for a walk when it started to rain. He did not have an umbrella and he wasn't wearing a hat. His clothes were soaked, yet not a single hair on his head got wet. How could this happen?

On her drawing table, Sayna lit 12 coloured candles. A few seconds later she blew two candles off.

How many candles Sayna have at the end?

I am a type of food that is often found in the fridge and can be white in colour. What I am?
Hint 1: I am a dairy product.
Hint 2: You can spread me on the bread.

I am a type of vehicle that has two wheels and is powered by humans. What am I?
Hint 1: You can ride me for exercise.
Hint 2: I am environment-friendly.

In four steps you need to move the word 'cold' From 'warm' by replacing one alphabet at a time such that every word formed at each step is acceptable in English Dictionary.