How do I make my keyboard show numbers and letters at the same time?

Your keyboard technically can give you numbers and letters at the same time. When you are typing in iOS 11, you can swipe down on the upper row of letters to add numbers. You could also add a different keyboard from a third-party source. See this article for more information on keyboards in general: About the keyboards settings on your iPhone, iPad, and iPod touch - Apple Support

zfJames wrote:

Your keyboard technically can give you numbers and letters at the same time. When you are typing in iOS 11, you can swipe down on the upper row of letters to add numbers.

Only on an iPad, not on an iPhone.