Whenever you need some quick, ad-hoc information about some numerical data in your sheet, there’s some information that Google Sheets automatically makes available.
Simply highlight the range of cells you’re interested in, and look to the bottom right of the spreadsheet window:
You’ll see the sum of the values in the range displayed in a small window. Sometimes that’s all you need, but click on that window to get more information about the data in question, including minimum, maximum, average value and count of items:
Note, however, that this feature is enabled based on your location, and it may not work for some numeric or currency formats.