1. You should wear a swimming cap in the swimming pool.

2. You should wear a swimming cap at the swimming pool.

(Which preposition should we use? Are both OK?)

3. What is your number?
- My number is 22.

4. May I have your number?
- My number is 22.

5. What number are you?
- I am 22.
(Are all the questions and answers correct in Sentence 3 to Sentence 5?)

The better preposition is "in."

Everything else is correct.

1. Both prepositions "in" and "at" can be used in this context, but their meanings may slightly differ.

Using "in" implies that you should wear a swimming cap while you are inside the pool, actually swimming in the water.

On the other hand, using "at" suggests that you should wear a swimming cap while you are in the vicinity of the pool, such as when you are sitting by the poolside, getting ready to swim, or even just being present at the pool area.

Both prepositions are generally acceptable, but the choice depends on the specific context and the intended meaning.

2. The questions and answers in sentences 3 to 5 are correct, with one small clarification.

In sentence 3, the question "What is your number?" is asking for a specific number, such as a phone number or an identification number. The answer "My number is 22" implies that 22 is the desired number.

In sentence 4, the question "May I have your number?" is also asking for a specific number, most commonly a phone number or any other contact details. The answer "My number is 22" suggests that the person is willing to share the number 22.

In sentence 5, the question "What number are you?" is asking for a numerical representation of an individual, usually used in a group or ranking context. The answer "I am 22" indicates that the person's position or ranking is 22.

Overall, the questions and answers make sense and are grammatically correct.