All the cricket players in the world cup have different numbers on their caps. What are they for?

International cricket isn’t about ability anymore, it’s about being mentally strong enough and getting money.
The way cricket’s going is it soon going to be a thing of the past?

How many different cricket teams can be chosen from a list of 17 players? My book says 12,376 but doesn’t show me how to do it.
How do I do it without using a calculator, please? Many thanks. I know people today use calculators but this book was written years ago so how did people do it without them? Thanks.