Can you give a specific example from your own setup? In a single numeric field there wouldn’t be multiple values, so I’m not sure what you really mean by “the values in a numeric field”.
If you have values in multiple numeric fields and want to add them together, create a formula field. List each of the fields containing the values that you want to add (selecting them from the popup formula helper will be far easier than typing them), and put + signs between each of them, like so:
Gotcha. The easiest way to do that across all records in a given view (or group if you’re grouping records) is to look at the summary bar. If you’re viewing your table full screen, it’s easy to overlook because it’s at the very bottom of the screen. By default for most numeric fields, it’s set to Sum mode, which adds up all values (numeric, currency, etc) for each field. It also list the total number of records at its far left edge. Here’s a compacted view (I manually shrank my window to make it easier to see) that shows the summary bar below a number of records:
Click on any of those “Sum” entries and you can change the mode depending on the field type.